Miscellany

Jai Ganesh · 2021-03-06 00:10:32

929) Esophageal cancer

Esophageal cancer, disease characterized by the abnormal growth of cells in the esophagus, the muscular tube connecting the oral cavity with the stomach. There are two types of esophageal cancer: squamous cell carcinoma, which develops from epithelial cells lining the esophagus, and adenocarcinoma, which originates in glandular cells. While squamous cell carcinoma accounts for the majority of cases of esophageal cancer globally, adenocarcinoma is on the rise in Western countries. Men are more than twice as likely to develop esophageal cancer as women, and, while blacks are more likely to develop squamous cell carcinoma than whites, whites are more likely to develop adenocarcinoma.

Causes And Symptoms

Several risk factors have been identified that increase the likelihood of developing esophageal cancer. Some factors, such as age, gender, and race, are impossible to control. However, tobacco and alcohol use increase risk, and these behaviours can be controlled. People who accidently swallowed lye as children also have a higher risk of esophageal cancer as adults. Long-term problems with acid reflux may lead to a condition called Barrett’s esophagus, in which the normal squamous cells that line the esophagus are replaced with glandular cells; this condition increases cancer risk. Rare disorders such as tylosis, achalasia, and Plummer-Vinson syndrome are also risk factors.

Esophageal cancers are usually diagnosed once symptoms have appeared, but by this time the cancer has usually developed to a relatively advanced stage. Symptoms may include difficulty or pain when swallowing, pain or tightness in the chest, unexplained weight loss, hoarseness, or frequent hiccups.

Diagnosis And Prognosis

If cancer is suspected, a thorough examination is conducted to determine its type and stage. The esophagus is visually examined with an endoscope, and tissue samples are taken for biopsy. Several imaging methods are frequently used, such as chest X rays, computed tomography (CT) scans, or ultrasound. There is no definitive laboratory test for esophageal cancer.

Once esophageal cancer has been diagnosed, its stage is determined to indicate how far the cancer has progressed. Stage 0 esophageal cancer is also called carcinoma in situ and is confined to the inner layer of epithelial cells lining the esophagus. Stage I cancers have spread into the connective tissue layer below the epithelium but have not invaded the underlying muscle layer. Stage II cancers either have spread through the muscle layer to the outer boundaries of the esophagus or have spread only into the muscle layer but have reached nearby lymph nodes. Stage III esophageal cancers have spread through the esophageal wall to the lymph nodes or other local tissues. Stage IV cancers have metastasized, or spread, to distant organs such as the stomach, liver, bone, or brain.

The survival rate for esophageal cancer is lower than for many other cancers. When the cancer is detected before it has invaded the underlying tissue layers of the esophagus, five-year survival is high, but fewer than 25 percent of esophageal cancers are diagnosed at this stage. If the cancer has moved to the tissue immediately underlying the mucosal surface, five-year survival is reduced to about 50 percent, and the rate drops significantly once the cancer has moved from the esophagus to nearby lymph nodes or other tissues. Once the cancer has spread to distant tissues in the body, five-year survival is extremely low.

Treatment

Esophageal cancers are best treated surgically when possible. If the cancer is confined to the upper region of the esophagus, an esophagectomy may be done to remove the cancerous portion, along with nearby lymph nodes, and to reconnect the remaining esophagus to the stomach. For cancers of the lower esophagus, it may be necessary to perform an esophagogastrectomy, in which a portion of the esophagus is removed along with a portion of the stomach. The stomach is then reattached directly to the remaining esophagus, or a segment of the colon is used to link the stomach and esophagus. Both of these surgeries are difficult and often result in serious complications. Other, less-drastic surgeries may be used to relieve symptoms, especially when surgical cure is not possible.

Treatment with radiation alone does not cure esophageal cancer, but it may be used either before surgery to shrink the size of the tumour or following surgery to destroy remaining cancer cells. Radiation therapy is also used to relieve symptoms. The side effects of radiation treatment include vomiting, diarrhea, fatigue, and esophageal irritation. Chemotherapy is also used for some esophageal cancers. It is not curative, but it can relieve some symptoms and may be able to shrink tumours prior to surgery. Side effects resemble those of radiotherapy.

Prevention

Esophageal cancer cannot be completely prevented, but risk can be lowered by reducing alcohol consumption and avoiding tobacco. Individuals who are at high risk should receive regular screening in order to increase the probability of early detection. Because there is no blood test available for esophageal cancer, screening requires regular biopsies and viewing of the esophagus with an endoscope.

Jai Ganesh · 2021-03-07 00:13:47

930) Platelet

Platelet, also called thrombocyte, colourless, nonnucleated blood component that is important in the formation of blood clots (coagulation). Platelets are found only in the blood of mammals.

Platelets are formed when cytoplasmic fragments of megakaryocytes, which are very large cells in the bone marrow, pinch off into the circulation as they age. They are stored in the spleen. Some evidence suggests platelets may also be produced or stored in the lungs, where megakaryocytes are frequently found.

Platelets play an important role in the formation of a blood clot by aggregating to block a cut blood vessel and provide a surface on which strands of fibrin form an organized clot, by contracting to pull the fibrin strands together to make the clot firm and permanent, and, perhaps most important, by providing or mediating a series of clotting factors necessary to the formation of the clot. Platelets also store and transport several chemicals, including serotonin, epinephrine, histamine, and thromboxane; upon activation these molecules are released and initiate local blood vessel constriction, which facilitates clot formation.

At birth the number of platelets is low, but by three months of age the adult level is reached. The number of platelets rises following trauma or asphyxiation, at high altitudes, after exercise, and in cold temperatures; the number may be temporarily lowered by menstruation in women. Certain chemicals may prolong the life of platelets; smoking is believed to shorten their life spans.

Jai Ganesh · 2021-03-08 00:19:46

931) Carcinogen

Carcinogen, any of a number of agents that can cause cancer in humans. They can be divided into three major categories: chemical carcinogens (including those from biological sources), physical carcinogens, and oncogenic (cancer-causing) viruses.

Most carcinogens, singly or in combination, produce cancer by interacting with DNA in cells and thereby interfering with normal cellular function. This ultimately results in the formation of a tumour (an abnormal tissue growth) that has the ability to spread (metastasize) from its site of origin and invade and cause dysfunction of other tissues, culminating in organ failure and death. The two primary mechanisms by which carcinogens initiate the formation of such tumours is via alterations in DNA that encourage cell division and that prevent cells from being able to self-destruct when stimulated by normal triggers, such as DNA damage or cellular injury (a process known as apoptosis). There also exist carcinogens that induce cancer through nongenotoxic mechanisms, such as immunosuppression and induction of tissue-specific inflammation.

More than 400 chemical agents have been listed as carcinogenic, probably carcinogenic, or possibly carcinogenic by the International Agency for Research on Cancer (IARC), a branch of the World Health Organization that monitors cancer occurrence worldwide and performs epidemiological and laboratory investigations to understand the causes of cancer. Among the carcinogenic substances listed by IARC are a variety of chemical effluents from industry and environmental pollutants from automobiles, residences, and factories. One such example is acrylamide, which is considered a probable carcinogen in humans and is produced as a result of industrial processes and cooking certain foods at high temperatures. It can be released into the environment through its application in wastewater treatment and its use in grout and soil-stabilizer products. Other examples of chemical carcinogens include nitrosamines and polycyclic aromatic hydrocarbons, which are found in tobacco smoke and are associated with the development of lung cancer.

Physical carcinogens include ultraviolet rays from sunlight and ionizing radiation from X-rays and from radioactive materials in industry and in the general environment. Repeated local injury (e.g., wounding) or recurring irritation (e.g., chronic inflammation) to a part of the body are other examples of potential physical carcinogens.

A number of viruses are suspected of causing cancer in animals, including humans, and are frequently referred to as oncogenic viruses. Examples include human papillomaviruses, the Epstein-Barr virus, and the hepatitis B virus, all of which have genomes made up of DNA. Human T-cell leukemia virus type I (HTLV-I), which is a retrovirus (a type of RNA virus), is linked to tumour formation in humans.

Some—not all—cancers are heritable in the sense that a predisposition exists, awaiting a convergence of carcinogenic influences for cancer to manifest itself. The identification and timely elimination of carcinogens can reduce the incidence of cancer.

Jai Ganesh · 2021-03-09 00:27:12

932) Brain cancer

Brain cancer, the uncontrolled growth of cells in the brain. The term brain cancer refers to any of a variety of tumours affecting different brain cell types. Depending on the location and cell type, brain cancers may progress rapidly or slowly over a period of many years. Brain cancers are often difficult to treat, and complete cure is often unattainable.

Causes And Symptoms

The causes of different brain cancers remain largely unknown. However, researchers have identified several risk factors, including exposure to ionizing radiation, such as head X-rays at therapeutic doses (as opposed to diagnostic doses); suppression of the immune system, which may be associated with immunosuppressive therapy or with an immunodeficiency disorder (particularly one that is inherited); and family history of cancer. Symptoms of brain cancer vary widely depending on the location of the tumour. As the tumour grows, it might put pressure on nearby regions of the brain and thereby affect the functions controlled by those regions. Difficulty or changes in speech, hearing, vision, or motor functions can all indicate the presence of a brain tumour. Many brain tumours are initially discovered following chronic headaches, and in some cases seizures are associated with cancers of the brain. Symptoms may also include vomiting, nausea, or numbness in any part of the body.

Diagnosis And Prognosis

If a brain tumour is suspected, a neurological exam is conducted to test general brain function. Further diagnosis usually utilizes imaging procedures such as X-rays, computed tomography (CT) scans, and magnetic resonance imaging (MRI). The location and stage of a tumour can also be determined with positron emission tomography (PET) scans. The blood supply feeding a tumour can be assessed by using an X-ray procedure called angiography. A definitive diagnosis usually requires removal of brain tissue for analysis; often this is done during tumour-removal surgery. In other cases, a needle biopsy guided by the images generated by CT scans or MRI may be used to access the tumour.

Brain cancers are usually not diagnosed until symptoms have appeared, and survival rates vary widely, depending on type and location. Some are completely curable. Slow-growing cancers may progress for decades, whereas other types may be fatal within six to eight years. Average survival from some faster-growing tumours, however, averages no more than one year.

Treatment

Surgery is the most frequent approach to treating brain tumours. Such surgery may be curative for some cancers, but for others it may only relieve symptoms and prolong survival. In many cases, complete removal of the tumour is not possible.

Radiation therapy may be used to cure some brain cancers, but others do not respond to radiotherapy. Radiation generally works best with fast-growing types. Because radiation therapy typically involves X-rays, which pose a risk to healthy brain tissue, it is important to minimize exposure to the normal cells surrounding the tumour. This is accomplished by employing special procedures that focus the radiation. For instance, a device called a gamma knife, which emits a highly controllable beam of radiation, may be used. Even when radiation is localized, however, radiotherapy can cause side effects such as vomiting, diarrhea, or skin irritation. Radiation to the brain may cause scar tissue to form and potentially cause future problems. Memory loss may also occur.

Chemotherapy is used for some brain tumours, but, owing to the brain’s protective barrier, many chemotherapeutic agents cannot enter the brain from the bloodstream. Chemotherapy works best on fast-growing tumours, but it is generally not curative and causes side effects similar to radiation therapy. Both radiation therapy and chemotherapy are often used when a person’s general health or the location of the tumour prevents surgery.

Prevention

In rare cases where a family history or a personal history of frequent head X-rays suggests an increased risk of brain cancer, regular screening by a neurologist may allow developing cancers to be detected earlier. Otherwise, no means of preventing brain tumours are known.

Jai Ganesh · 2021-03-10 00:27:12

933) Bone cancer

Bone cancer, disease characterized by uncontrolled growth of cells of the bone. Primary bone cancer—that is, cancer that arises directly in the bone—is relatively rare. In the United States, for example, only about 2,400 new cases of primary bone cancer are diagnosed each year. Most cancer that involves the bone is cancer that has spread (metastasized) from other tissues in the body through the blood or lymphatic systems. Different types of bone tissue give rise to different types of primary bone cancer. Osteosarcoma develops from cells that form the bone, and Ewing tumour of the bone (Ewing sarcoma) develops from immature nerve tissue within the bone. Both types most commonly affect males between 10 and 20 years of age. Chondrosarcoma, which forms in cartilage tissue, principally affects persons over age 50. More than one-half of the cases of primary bone cancer, even once-deadly types, can now be treated successfully.

Causes And Symptoms

Only a small portion of bone cancer cases are associated with known risk factors, which include exposure to radiation or chemotherapy, Paget disease, and rare hereditary syndromes such as hereditary retinoblastoma. The majority of cases seem to occur randomly in otherwise healthy individuals.

The most common symptom of bone cancer is pain or tenderness over the affected bone. Bone tumours often are not noticed until minor trauma causes significant pain and disability that leads to further investigation. This association has led to the mistaken conclusion that traumatic injuries can cause bone cancer. Other symptoms that can occur include bone fractures, decreased mobility of a joint, fever, fatigue, and anemia. These symptoms are not specific to bone cancer and can be the result of other, benign processes.

Diagnosis And Prognosis

Preliminary investigation of a bone tumour can include a blood test for the enzyme alkaline phosphatase. As bone cancer grows, the amount of the enzyme in the blood increases dramatically, but it can also increase for other reasons. With bone cancer, unlike many other types of cancer, X-ray imaging can be very helpful in making a diagnosis. The images will show whether a tumour is creating bone tissue or destroying normal bone tissue. Images of the bone useful for making a diagnosis can also be obtained by computed tomography (CT scans), magnetic resonance imaging (MRI), and a type of radioisotope scanning commonly called a bone scan. The final diagnosis of cancer, however, requires the removal of a portion of the tumour for examination under a microscope.

The prognosis of bone cancer depends on both the type of cancer and the extent to which it has spread. Bone cancer most frequently spreads to the lungs, but it may also spread to other bones and only rarely to other tissues. Overall, the prognosis for long-term survival has improved to more than 50 percent, including cases in which the tumour has spread to other parts of the body. Of the different types of primary bone cancer, chondrosarcoma has the best prognosis and osteosarcoma the worst.

Treatment

As with many cancers, the treatment of bone cancer depends on the type of cell, location, size, and spread of the primary tumour. Most cases require a combination of surgery, chemotherapy, and radiation. In some cases, surgery requires the amputation of the involved limb. In other cases, it may be possible to remove only a portion of the bone and replace it with a prosthesis or bone graft. Chemotherapy may be given before or after surgery and is tailored to the specific type of bone cancer.

Prevention

Prevention of bone cancer will require a better understanding of its causes than is currently available. If a patient has a known risk factor for bone cancer, such as Paget disease, careful screening may help detect and treat the cancer in its early stages, thereby improving the chances for survival.

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Jai Ganesh · 2021-03-11 00:33:40

934) Natural fibre

Natural fibre, any hairlike raw material directly obtainable from an animal, vegetable, or mineral source and convertible into nonwoven fabrics such as felt or paper or, after spinning into yarns, into woven cloth. A natural fibre may be further defined as an agglomeration of cells in which the diameter is negligible in comparison with the length. Although nature abounds in fibrous materials, especially cellulosic types such as cotton, wood, grains, and straw, only a small number can be used for textile products or other industrial purposes. Apart from economic considerations, the usefulness of a fibre for commercial purposes is determined by such properties as length, strength, pliability, elasticity, abrasion resistance, absorbency, and various surface properties. Most textile fibres are slender, flexible, and relatively strong. They are elastic in that they stretch when put under tension and then partially or completely return to their original length when the tension is removed.

History

The use of natural fibres for textile materials began before recorded history. The oldest indication of fibre use is probably the discovery of flax and wool fabrics at excavation sites of the Swiss lake dwellers (7th and 6th centuries BCE). Several vegetable fibres were also used by prehistoric peoples. Hemp, presumably the oldest cultivated fibre plant, originated in Southeast Asia, then spread to China, where reports of cultivation date to 4500 BCE. The art of weaving and spinning linen was already well developed in Egypt by 3400 BCE, indicating that flax was cultivated sometime before that date. Reports of the spinning of cotton in India date back to 3000 BCE. The manufacture of silk and silk products originated in the highly developed Chinese culture; the invention and development of sericulture (cultivation of silkworms for raw-silk production) and of methods to spin silk date from 2640 BCE.

With improved transportation and communication, highly localized skills and arts connected with textile manufacture spread to other countries and were adapted to local needs and capabilities. New fibre plants were also discovered and their use explored. In the 18th and 19th centuries, the Industrial Revolution encouraged the further invention of machines for use in processing various natural fibres, resulting in a tremendous upsurge in fibre production. The introduction of regenerated cellulosic fibres (fibres formed of cellulose material that has been dissolved, purified, and extruded), such as rayon, followed by the invention of completely synthetic fibres, such as nylon, challenged the monopoly of natural fibres for textile and industrial use. A variety of synthetic fibres having specific desirable properties began to penetrate and dominate markets previously monopolized by natural fibres. Recognition of the competitive threat from synthetic fibres resulted in intensive research directed toward the breeding of new and better strains of natural-fibre sources with higher yields, improved production and processing methods, and modification of fibre yarn or fabric properties. The considerable improvements achieved have permitted increased total production, although natural fibres’ actual share of the market has decreased with the influx of the cheaper, synthetic fibres requiring fewer man-hours for production.

Classification And Properties

Natural fibres can be classified according to their origin. The vegetable, or cellulose-base, class includes such important fibres as cotton, flax, and jute. The animal, or protein-base, fibres include wool, mohair, and silk. An important fibre in the mineral class is asbestos.

The vegetable fibres can be divided into smaller groups based on their origin within the plant. Cotton, kapok, and coir are examples of fibres originating as hairs borne on the seeds or inner walls of the fruit, where each fibre consists of a single, long, narrow cell. Flax, hemp, jute, and ramie are bast fibres, occurring in the inner bast tissue of certain plant stems and made up of overlapping cells. Abaca, henequen, and sisal are fibres occurring as part of the fibrovascular system of the leaves. Chemically, all vegetable fibres consist mainly of cellulose, although they also contain varying amounts of such substances as hemicellulose, lignin, pectins, and waxes that must be removed or reduced by processing.

The animal fibres consist exclusively of proteins and, with the exception of silk, constitute the fur or hair that serves as the protective epidermal covering of animals. Silk filaments are extruded by the larvae of moths and are used to spin their cocoons.

With the exception of mineral fibres, all natural fibres have an affinity for water in both liquid and vapour form. This strong affinity produces swelling of the fibres connected with the uptake of water, which facilitates dyeing in watery solutions.

Unlike most synthetic fibres, all natural fibres are nonthermoplastic; that is, they do not soften when heat is applied. At temperatures below the point at which they will decompose, they show little sensitivity to dry heat, and there is no shrinkage or high extensibility upon heating, nor do they become brittle if cooled to below freezing. Natural fibres tend to yellow upon exposure to sunlight and moisture, and extended exposure results in loss of strength.

All natural fibres are particularly susceptible to microbial decomposition, including mildew and rot. Cellulosic fibres are decomposed by aerobic bacteria (those that live only in oxygen) and fungi. Cellulose mildews and decomposes rapidly at high humidity and high temperatures, especially in the absence of light. Wool and silk are also subject to microbial decomposition by bacteria and molds. Animal fibres are also subject to damage by moths and carpet beetles. Termites and silverfish attack cellulose fibres. Protection against both microbial damage and insect attacks can be obtained by chemical modification of the fibre substrate; modern developments allow treatment of natural fibres to make them essentially immune to such damage.

Jai Ganesh · 2021-03-12 00:18:52

935) Lapland

Lapland, Sami Sápmi, Finnish Lapi or Lappi, Swedish Lappland, region of northern Europe largely within the Arctic Circle, stretching across northern Norway, Sweden, and Finland and into the Kola Peninsula of Russia. It is bounded by the Norwegian Sea on the west, the Barents Sea on the north, and the White Sea on the east. Lapland, the conventional name for the region, is derived from Lapp, the name Scandinavians ascribed to the Sami people, who have sparsely inhabited the region for several thousand years. Today the Sami consider Lapp to be a derogatory term. They call the region Sápmi. Lapland straddles several national borders and does not exist as any unified administrative entity.

Lapland is a region of great topographical variety. To the west it embraces the northern part of the Kolen Mountains, which reach elevations of more than 6,500 feet (2,000 metres). On its Norwegian (western) side this range slopes abruptly and is deeply eroded into fjords and headlands and fractured into archipelagoes. The eastern flank of the range, which is situated in Swedish Lapland, slopes more gradually into a broad piedmont studded with large, fingerlike lakes that feed the rivers flowing into the Gulf of Bothnia. Farther to the east, Finnish Lapland (Lappi) is a relatively low-lying region with many bogs and small lakes.

Norwegian Lapland is largely open and windswept, with timber growth only in sheltered tracts and the more protected interior. Southern and central Lapland occupies the zone of the taiga, or swampy coniferous forest, with its saturated land and many bogs and swamps. Forests of pine and spruce give way to the dwarf birch, heath, and lichens of the tundra farther north and at higher elevations.

Many of the Sami have adopted a sedentary life and intermarried with Scandinavians and Finns. The region is still home to several hundred thousand reindeer, but the traditional reindeer country has been intruded upon by permanent farming, forestry, mining, and hydroelectric and even industrial enterprises. Those who practice reindeer herding have liberty of movement across the open boundaries of Finland, Norway, and Sweden.

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Jai Ganesh · 2021-03-13 00:28:58

936) Beriberi

Beriberi, nutritional disorder caused by a deficiency of thiamin (vitamin B1) and characterized by impairment of the nerves and heart. General symptoms include loss of appetite and overall lassitude, digestive irregularities, and a feeling of numbness and weakness in the limbs and extremities. (The term beriberi is derived from the Sinhalese word meaning “extreme weakness.”) In the form known as dry beriberi, there is a gradual degeneration of the long nerves, first of the legs and then of the arms, with associated atrophy of muscle and loss of reflexes. In wet beriberi, a more acute form, there is edema (overabundance of fluid in the tissues) resulting largely from cardiac failure and poor circulation. In infants breast-fed by mothers who are deficient in thiamin, beriberi may lead to rapidly progressive heart failure.

The cardiac symptoms, in both infants and adults, generally respond promptly and dramatically to the administration of thiamin. When neurological involvement is present, the response to thiamin is much more gradual; in severe cases, the structural lesions of the nerve cells may be irreversible.

Thiamin normally plays an essential role as a coenzyme in the metabolism of carbohydrates; in its absence, pyruvic acid and lactic acid (products of carbohydrate digestion) accumulate in the tissues, where they are believed to be responsible for most of the neurological and cardiac manifestations.

Thiamin occurs widely in food but may be lost in the course of processing, particularly in the milling of grains. In East Asian countries, where polished white rice is a dietary staple, beriberi has been a long-standing problem. The history of the recognition, the cause, and the cure of beriberi is dramatic and is well documented in medical literature. In the 1880s the Japanese navy reported that beriberi had been eradicated among its sailors as a result of adding extra meat, fish, and vegetables to their regular diet. Before that time, almost half of the sailors were likely to develop beriberi, and many died of it. In 1897 Christiaan Eijkman, working in the Dutch East Indies (now Indonesia), found that a beriberi-like disease could be produced in chickens by feeding them a diet of polished rice. British researchers William Fletcher, Henry Fraser, and A.T. Stanton later confirmed that beriberi in humans was also related to the consumption of polished white rice. In 1912 Casimir Funk demonstrated that beriberi-like symptoms induced in pigeons could be cured by feeding them white rice that was supplemented with a concentrate made from rice polishings. Following this discovery he proposed that this, as well as several other conditions, were due to diets that were deficient in specific factors that he called “vitamines,” later called vitamins.

The prevention of beriberi is accomplished by eating a well-balanced diet, since thiamin is present in most raw and untreated foods. The incidence of beriberi in Asia has markedly decreased because an improved standard of living has allowed a more varied diet and partly because of the gradual popular acceptance of partially dehusked, parboiled, and enriched rice—forms that contain higher concentrations of thiamin. In Western countries, thiamin deficiency is encountered almost solely in cases of chronic alcoholism.

Jai Ganesh · 2021-03-14 00:44:39

937) Estuary

Estuary, partly enclosed coastal body of water in which river water is mixed with seawater. In a general sense, the estuarine environment is defined by salinity boundaries rather than by geographic boundaries. The term estuary is derived from the Latin words aestus (“the tide”) and aestuo (“boil”), indicating the effect generated when tidal flow and river flow meet.

Estuaries are places where rivers meet the sea and may be defined as areas where salt water is measurably diluted with fresh water. On average, estuaries are biologically more productive than either the adjacent river or the sea, because they have a special kind of water circulation that traps plant nutrients and stimulates primary production. Fresh water, being lighter than salt water, tends to form a distinct layer that floats at the surface of the estuary. At the boundary between fresh and salt water, there is a certain amount of mixing caused by the flow of fresh water over salt and by the ebb and flow of tides. Additional mixing may be caused from time to time by strong winds and by internal waves that are propagated along the interface between fresh and salt water.

Many coastal features that are designated by other names are in fact estuaries. For example, various coastal embayments, such as Chesapeake Bay and Galveston Bay in the United States, also are estuaries, because fresh and salt water undergo considerable mixing. Moreover, most of the world’s submerged fjord systems (such as Scoresby Sund, Greenland) and large semi-enclosed tidal flat regions and coastal marshes (such as the Waddenzee area of The Netherlands) are estuaries. In addition, estuaries include river mouths (as in the case of the Mississippi River, Louisiana), structural basins (San Francisco Bay, California), and the bodies of water behind spits (Hurst Castle spit, England) and barrier beaches (Ninety Mile Beach, Australia). In the case of spits and barrier beaches, the definitions of lagoons and estuaries overlap.

Types

The four basic types of estuaries are (1) the salt wedge estuary, (2) the partially mixed (or slightly stratified) estuary, (3) the vertically homogeneous (or vertically mixed) estuary, and (4) the fjord (or highly stratified estuary).

Salt wedge estuaries

A salt wedge estuary has minimal mixing, and the salt water forms a wedge, thickest at the seaward end, tapering to a very thin layer at the landward limit. The penetration of this wedge changes with the flow of the river. During flood conditions the wedge will retreat, and during low flows it will extend farther upriver. The mouth of the Mississippi River in the United States is a classic example. The mixing at the boundary between fresh and salt water causes the surface layer to entrain salt water and become more saline as it moves toward the sea. To compensate for the entrained salt water, there is a slow movement of the salt water up the estuary at depth. Because bottom waters are rich in nutrients derived from decomposing plant and animal remains, this circulation has the effect of pumping nutrients into the estuary and stimulating biological production.

Organic and inorganic particles carried by rivers tend to flocculate (aggregate into a mass) and sediment out when they encounter salt water. They sink from the freshwater layer to the salt wedge and are carried upstream. When the organic matter decomposes, it adds still more nutrients to the estuary. The inorganic matter settles on the bottom and provides an enriched sediment for flowering plants adapted to salt water. Between the tide marks, mangrove forests flourish in tropical conditions, and salt marshes form in temperate and subarctic conditions. Below low tide, sea grasses form dense beds on muddy substrates. In areas of an estuary where water movement is vigorous enough to remove sediment, leaving a stony or rocky bottom, rooted plants are replaced by seaweeds. These have a special structure known as a holdfast, which attaches itself to any hard surface. Phytoplankton floating freely in the water benefit from the high level of nutrients, especially near the head of the estuary, and grow rapidly, providing food for the microscopic animals in the water column, the zooplankton. As this community is carried downstream in the surface waters, dead organisms and the fecal pellets of the animals sink toward the bottom and enter the salt wedge to be carried back to the head of the estuary. As they decompose, they add still more nutrients to the water.

Partially mixed estuaries

In a partially mixed estuary, the vigorous rise and fall of the tide generates strong turbulence and causes partial mixing between the fresh water above and the salt water below. Under these conditions the river flow entrains 10 to 20 or more times its own volume of salt water, and the compensatory landward flow of seawater near the bottom is correspondingly increased. The effect of Earth’s rotation (Coriolis effect) is to cause the surface flow to be stronger on the right-hand side facing seaward in the Northern Hemisphere and on the opposite side in the Southern Hemisphere. The bottom flow is stronger on the opposite side of the estuary.

Vertically homogeneous estuaries

In a vertically homogeneous estuary the river flow is weak, and the tidal flow is strong. Consequently, no stratification occurs, and salt water may even move up the river channel. Salinity levels are nearly the same from top to bottom at any given place in such estuaries; however, the salinity is lowest where the river enters the estuary and highest near the sea.

Fjords

The fjord-type estuary was originally formed by a glacier and has a submerged ridge, or sill, near its mouth, composed of glacial deposits. It may be regarded as a partially mixed estuary in which the bottom has been replaced by a basin of undiluted seawater held in place by the sill. When entrainment in river flow causes a strong landward flow at the bottom, water rises over the sill and enters the estuary at intermediate depth, leaving the deep waters undisturbed. Only major intrusions of seawater caused by storms can displace the deep water. Fjords, because of their glacial origin, commonly have steep sides and very little shallow water. Hence, the development of salt marshes or sea grass beds is minimal, but seaweeds colonize the rocky shores.

Geology And Geomorphology

The geologic processes that form an estuary are extremely complex and varied, but it is clear that the existence of an estuary is largely dependent on the position of sea level relative to the freshwater discharge. If sea level were lowered, the estuarine zone would migrate seaward at the interface of the marine water and the edge of the newly exposed land area. Such migration has occurred as a consequence of Earth’s several glaciations. For each glaciation, the primary source of moisture has been the oceans. Whenever sea level fell, the estuarine environment at the continental margin was forced to migrate seaward.

About 18,000 years ago the Wisconsin Glacial Stage attained its maximum, and glacial melting began. The seas rose, forcing the estuarine environment to migrate back up the continental shelf. During the period of lowered sea level, some rivers had become entrenched in the continental shelf and deepened their valleys, which were soon flooded by the rising marine waters, forming a typical drowned river estuary. In areas such as Norway and parts of the coast of British Columbia, Canada, valley glaciers had deepened river valleys. These narrow drowned glacial valleys became the modern fjord estuaries as sea level rose.

The geomorphology of an estuarine basin is usually developed by one of three agents: (1) fluvial or glacial erosion, (2) fluvial and marine deposition, or (3) tectonic activity. The last of these involves the downfaulting of a coastal area or the broad local subsidence of a stretch of coastline, as in the case of San Francisco Bay.

Primary Productivity

The high level of plant production in estuaries supports a correspondingly high level of production of invertebrate animals and fish. Estuaries often contain beds of shellfish such as mussels and oysters and large populations of shrimps and crabs. Fish such as plaice and flounders are common. Other species use the estuaries as nursery grounds. Organisms in early stages of development enter the salt wedge at the seaward end and are carried up the estuary by the bottom currents. Juveniles find abundant food as well as protection from predators in the mangrove forests, salt marshes, or sea grass beds that line the estuary. Later they may migrate to the open ocean to continue their growth and development. Other species pass through the estuaries in the course of their migrations. For example, salmon migrate from the sea to the rivers to spawn, and the young fish later migrate back to the sea. Eels migrate in the opposite direction, breeding in the sea but returning to fresh water as juveniles.

Commercial Importance

Estuaries have long been important as harbour sites and centres of commerce. Some of the world’s oldest continuous civilizations have flourished in estuarine environments, such as the lower reaches of the Tigris and Euphrates rivers, the Indus River, the Po River delta region of Italy, the Nile delta, the Ganges delta, and the lower Huang He (Yellow River) valley. Developing civilizations soon discovered that the logical site for commercial seaports was the seawardmost point of the major river systems. Such cities as London (River Thames), New York City (Hudson River), Montreal (St. Lawrence River), Hamburg (Elbe River), and Bordeaux (Gironde estuary) developed on estuaries and became important centres of commerce. The proximity of estuaries to large urban centres, however, has made them particularly vulnerable to contamination by sewage and industrial effluents. The characteristic circulation that serves to trap natural plant nutrients may also retain high concentrations of pollutants.

In addition, many estuaries are important sites for aquaculture. There is a long history of mussel culture along the coast of Spain, and Norwegian fjords are much used for salmon culture. In Southeast Asia artificial ponds are created in mangrove forests and are used to culture shrimp.

Jai Ganesh · 2021-03-15 00:27:39

938) Nitrogen fixation

Nitrogen fixation, any natural or industrial process that causes free nitrogen (N2), which is a relatively inert gas plentiful in air, to combine chemically with other elements to form more-reactive nitrogen compounds such as ammonia, nitrates, or nitrites.

Under ordinary conditions, nitrogen does not react with other elements. Yet nitrogenous compounds are found in all fertile soils, in all living things, in many foodstuffs, in coal, and in such naturally occurring chemicals as sodium nitrate (saltpetre) and ammonia. Nitrogen is also found in the nucleus of every living cell as one of the chemical components of DNA.

Nitrogen Fixation In Nature

Nitrogen is fixed, or combined, in nature as nitric oxide by lightning and ultraviolet rays, but more significant amounts of nitrogen are fixed as ammonia, nitrites, and nitrates by soil microorganisms. More than 90 percent of all nitrogen fixation is effected by them. Two kinds of nitrogen-fixing microorganisms are recognized: free-living (nonsymbiotic) bacteria, including the cyanobacteria (or blue-green algae) Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium; and mutualistic (symbiotic) bacteria such as Rhizobium, associated with leguminous plants, and various Azospirillum species, associated with cereal grasses.

The symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate the formation of root nodules, enlargements of plant cells and bacteria in intimate association. Within the nodules, the bacteria convert free nitrogen to ammonia, which the host plant utilizes for its development. To ensure sufficient nodule formation and optimum growth of legumes (e.g., alfalfa, beans, clovers, peas, and soybeans), seeds are usually inoculated with commercial cultures of appropriate Rhizobium species, especially in soils poor or lacking in the required bacterium.

Industrial Nitrogen Fixation

Nitrogenous materials have long been used in agriculture as fertilizers, and in the course of the 19th century the importance of fixed nitrogen to growing plants was increasingly understood. Accordingly, ammonia released in making coke from coal was recovered and utilized as a fertilizer, as were deposits of sodium nitrate (saltpetre) from Chile. Wherever intensive agriculture was practiced, there arose a demand for nitrogen compounds to supplement the natural supply in the soil. At the same time, the increasing quantity of Chile saltpetre used to make gunpowder led to a worldwide search for natural deposits of this nitrogen compound. By the end of the 19th century it was clear that recoveries from the coal-carbonizing industry and the importation of Chilean nitrates could not meet future demands. Moreover, it was realized that, in the event of a major war, a nation cut off from the Chilean supply would soon be unable to manufacture munitions in adequate amounts.

During the first decade of the 20th century, intensive research efforts culminated in the development of several commercial nitrogen-fixation processes. The three most-productive approaches were the direct combination of nitrogen with oxygen, the reaction of nitrogen with calcium carbide, and the direct combination of nitrogen with hydrogen. In the first approach, air or any other uncombined mixture of oxygen and nitrogen is heated to a very high temperature, and a small portion of the mixture reacts to form the gas nitric oxide. The nitric oxide is then chemically converted to nitrates for use as fertilizers. By 1902 electric generators were in use at Niagara Falls, New York, to combine nitrogen and oxygen in the high temperatures of an electric arc. This venture failed commercially, but in 1904 Christian Birkeland and Samuel Eyde of Norway used an arc method in a small plant that was the forerunner of several larger, commercially successful plants that were built in Norway and other countries.

The arc process, however, was costly and inherently inefficient in its use of energy, and it was soon abandoned for better processes. One such method used the reaction of nitrogen with calcium carbide at high temperatures to form calcium cyanamide, which hydrolyzes to ammonia and urea. The cyanamide process was utilized on a large scale by several countries before and during World War I, but it too was energy-intensive, and by 1918 the Haber-Bosch process had rendered it obsolete.

The Haber-Bosch process directly synthesizes ammonia from nitrogen and hydrogen and is the most economical nitrogen-fixation process known. About 1909 the German chemist Fritz Haber ascertained that nitrogen from the air could be combined with hydrogen under extremely high pressures and moderately high temperatures in the presence of an active catalyst to yield an extremely high proportion of ammonia, which is the starting point for the production of a wide range of nitrogen compounds. This process, made commercially feasible by Carl Bosch, came to be called the Haber-Bosch process or the synthetic ammonia process. Germany’s successful reliance on this process during World War I led to a rapid expansion of the industry and the construction of similar plants in many other countries after the war. The Haber-Bosch method is now one of the largest and most-basic processes of the chemical industry throughout the world.

Nitrogen%20Fixation.png

Jai Ganesh · 2021-03-16 00:09:55

939) Nitrogen cycle

Nitrogen cycle, circulation of nitrogen in various forms through nature. Nitrogen, a component of proteins and nucleic acids, is essential to life on Earth. Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms. Through a series of microbial transformations, however, nitrogen is made available to plants, which in turn ultimately sustain all animal life. The steps, which are not altogether sequential, fall into the following classifications: nitrogen fixation, nitrogen assimilation, ammonification, nitrification, and denitrification.

Nitrogen fixation, in which nitrogen gas is converted into inorganic nitrogen compounds, is mostly (90 percent) accomplished by certain bacteria and blue-green algae. A much smaller amount of free nitrogen is fixed by abiotic means (e.g., lightning, ultraviolet radiation, electrical equipment) and by conversion to ammonia through the Haber-Bosch process.

Nitrates and ammonia resulting from nitrogen fixation are assimilated into the specific tissue compounds of algae and higher plants. Animals then ingest these algae and plants, converting them into their own body compounds.

The remains of all living things—and their waste products—are decomposed by microorganisms in the process of ammonification, which yields ammonia (NH3) and ammonium (NH4+). (Under anaerobic, or oxygen-free, conditions, foul-smelling putrefactive products may appear, but they too are converted to ammonia in time.) Ammonia can leave the soil or be converted into other nitrogen compounds, depending in part on soil conditions.

Nitrification, a process carried out by nitrifying bacteria, transforms soil ammonia into nitrates (NO3−), which plants can incorporate into their own tissues.

Nitrates also are metabolized by denitrifying bacteria, which are especially active in water-logged anaerobic soils. The action of these bacteria tends to deplete soil nitrates, forming free atmospheric nitrogen.

Jai Ganesh · 2021-03-17 00:57:11

940) Oxygen cycle

Oxygen cycle, circulation of oxygen in various forms through nature. Free in the air and dissolved in water, oxygen is second only to nitrogen in abundance among uncombined elements in the atmosphere. Plants and animals use oxygen to respire and return it to the air and water as carbon dioxide (CO2). CO2 is then taken up by algae and terrestrial green plants and converted into carbohydrates during the process of photosynthesis, oxygen being a by-product. The waters of the world are the main oxygen generators of the biosphere; their algae are estimated to replace about 90 percent of all oxygen used. Oxygen is involved to some degree in all the other biogeochemical cycles. For example, over time, detritus from living organisms transfers oxygen-containing compounds such as calcium carbonates into the lithosphere.

Despite the burning of fossil fuel and the reduction of natural vegetation (on land and in the sea), the level of atmospheric oxygen appears to be relatively stable because of the increase in plant productivity resulting from agricultural advances worldwide.

Jai Ganesh · 2021-03-18 00:12:41

941) Phosphorus cycle

Phosphorus cycle, circulation of phosphorus in various forms through nature. Of all the elements recycled in the biosphere, phosphorus is the scarcest and therefore the one most limiting in any given ecological system. It is indispensable to life, being intimately involved in energy transfer and in the passage of genetic information in the deoxyribonucleic acid (DNA) of all cells.

Much of the phosphorus on Earth is tied up in rock and sedimentary deposits, from which it is released by weathering, leaching, and mining. Some of it passes through freshwater and terrestrial ecosystems via plants, grazers, predators, and parasites, to be returned to those ecosystems by death and decay. Much of it, however, is deposited in the sea, in shallow sediments, where it circulates readily, or in ocean deeps, whence it wells up only occasionally. Phosphorus is brought back to the land through fish harvests and through collection of guano deposited by seabirds. Although there are seasonal pulses of availability, there appears to be a steady loss of phosphorus to the ocean deeps.

Because of its high reactivity, phosphorus exists in combined form with other elements. Microorganisms produce acids that form soluble phosphate from insoluble phosphorus compounds. The phosphates are utilized by algae and terrestrial green plants, which in turn pass into the bodies of animal consumers. Upon death and decay of organisms, phosphates are released for recycling.

Because of the steady diversion of phosphorus into the oceans, the element must be added (in fertilizers) to soils to maintain fertility and agricultural productivity.

Jai Ganesh · 2021-03-19 00:40:16

942) Avogadro's law

Avogadro’s law, a statement that under the same conditions of temperature and pressure, equal volumes of different gases contain an equal number of molecules. This empirical relation can be derived from the kinetic theory of gases under the assumption of a perfect (ideal) gas. The law is approximately valid for real gases at sufficiently low pressures and high temperatures.

The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.02214076 × {10}^{23}, a quantity called Avogadro’s number, or the Avogadro constant. For example, the molecular weight of oxygen is 32.00, so that one gram-mole of oxygen has a mass of 32.00 grams and contains 6.02214076 × {10}^{23} molecules.

The volume occupied by one gram-mole of gas is about 22.4 litres (0.791 cubic foot) at standard temperature and pressure (0 °C, 1 atmosphere) and is the same for all gases, according to Avogadro’s law.

The law was first proposed in 1811 by Amedeo Avogadro, a professor of higher physics at the University of Turin for many years, but it was not generally accepted until after 1858, when an Italian chemist, Stanislao Cannizzaro, constructed a logical system of chemistry based on it.

Jai Ganesh · 2021-03-20 00:03:02

943) Troposphere

Troposphere, lowest region of the atmosphere, bounded by the Earth beneath and the stratosphere above, with its upper boundary being the tropopause, about 10–18 km (6–11 miles) above the Earth’s surface. The troposphere is characterized by decreasing temperature with height and is distinguished from the overlying stratosphere by a region of nearly constant temperature in the lower stratosphere. Most of the clouds and weather systems are contained within the troposphere.

The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains 75% of the atmosphere's mass and 99% of the total mass of water vapour and aerosols. The average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics, 17 km (11 mi; 56,000 ft) in the middle latitudes, and 6 km (3.7 mi; 20,000 ft) in the polar regions in winter. The total average height of the troposphere is 13 km (8.1 mi; 43,000 ft).

The lowest part of the troposphere, where friction with the Earth's surface influences airflow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi; 6,600 ft) deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.

The word troposphere is derived from the Greek tropos (meaning "turn, turn toward, change") and sphere (as in the Earth), reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena associated with day-to-day weather occur in the troposphere.

Composition:

By volume, dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.

Pressure

The pressure of the atmosphere is maximum at sea level and decreases with altitude. This is because the atmosphere is very nearly in hydrostatic equilibrium so that the pressure is equal to the weight of air above a given point.

Tropopause

The tropopause is the boundary region between the troposphere and the stratosphere.

Measuring the temperature change with height through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, temperature decreases with altitude. In the stratosphere, however, the temperature remains constant for a while and then increases with altitude. This coldest layer of the atmosphere, where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), is defined as the tropopause.[3] Thus, the tropopause is an inversion layer, and there is little mixing between the two layers of the atmosphere.

Atmospheric flow

The flow of the atmosphere generally moves in a west to east direction. This, however, can often become interrupted, creating a more north to south or south to north flow. These scenarios are often described in meteorology as zonal or meridional. These terms, however, tend to be used about localized areas of the atmosphere (at a synoptic scale).

Jai Ganesh · 2021-03-21 00:08:40

944) Stratosphere

Stratosphere, layer of Earth’s atmosphere lying between the troposphere and the mesosphere. The lower portion of the stratosphere is nearly isothermal (a layer of constant temperature), whereas temperatures in its upper levels increase with altitude. The stratosphere extends from the tropopause at about 10 to 17 km (about 6 to 11 miles) altitude to its upper boundary (the stratopause) at about 50 km (30 miles), and it also contains the ozone layer.

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. The stratosphere is stratified (layered) in temperature, with warmer layers higher and cooler layers closer to the Earth; this increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation (shortened UV) by the ozone layer. This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the lower edge of the stratosphere is as high as 20 km (66,000 ft; 12 mi), at midlatitudes around 10 km (33,000 ft; 6.2 mi), and at the poles about 7 km (23,000 ft; 4.3 mi). Temperatures range from an average of −51 °C (−60 °F; 220 K) near the tropopause to an average of −15 °C (5.0 °F; 260 K) near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near 60 m/s (220 km/h; 130 mph) in the Southern polar vortex.

Ozone and temperature

The mechanism describing the formation of the ozone layer was described by British mathematician Sydney Chapman in 1930. Molecular oxygen absorbs high energy sunlight in the UV-C region, at wavelengths shorter than about 240 nm. Radicals produced from the homolytically split oxygen molecules combine with molecular oxygen to form ozone. Ozone in turn is photolysed much more rapidly than molecular oxygen as it has a stronger absorption that occurs at longer wavelengths, where the solar emission is more intense. Ozone (O3) photolysis produces O and O2. The oxygen atom product combines with atmospheric molecular oxygen to reform O3, releasing heat. The rapid photolysis and reformation of ozone heats the stratosphere resulting in a temperature inversion. This increase of temperature with altitude is characteristic of the stratosphere; its resistance to vertical mixing means that it is stratified. Within the stratosphere temperatures increase with altitude; the top of the stratosphere has a temperature of about 270 K (−3°C or 26.6°F).

This vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. However, exceptionally energetic convection processes, such as volcanic eruption columns and overshooting tops in severe supercell thunderstorms, may carry convection into the stratosphere on a very local and temporary basis. Overall the attenuation of solar UV at wavelengths that damage DNA by the ozone layer allows life to exist on the surface of the planet outside of the ocean. All air entering the stratosphere must pass through the tropopause, the temperature minimum that divides the troposphere and stratosphere. The rising air is literally freeze dried; the stratosphere is a very dry place. The top of the stratosphere is called the stratopause, above which the temperature decreases with height.

Sydney Chapman gave a correct description of the source of stratospheric ozone and its ability to generate heat within the stratosphere; he also wrote that ozone may be destroyed by reacting with atomic oxygen, making two molecules of molecular oxygen. We now know that there are additional ozone loss mechanisms, and that these mechanisms are catalytic meaning that a small amount of the catalyst can destroy a great number of ozone molecules. The first is due to the reaction of hydroxyl radicals (•OH) with ozone. •OH is formed by the reaction of electronically excited oxygen atoms produced by ozone photolysis, with water vapor. While the stratosphere is dry, additional water vapor is produced in situ by the photochemical oxidation of methane (CH4). The HO2 radical produced by the reaction of OH with O3 is recycled to OH by reaction with oxygen atoms or ozone. In addition, solar proton events can significantly affect ozone levels via radiolysis with the subsequent formation of OH. Nitrous oxide (N2O) is produced by biological activity at the surface and is oxidised to NO in the stratosphere; the so-called NOx radical cycles also deplete stratospheric ozone. Finally, chlorofluorocarbon molecules are photolysed in the stratosphere releasing chlorine atoms that react with ozone giving ClO and O2. The chlorine atoms are recycled when ClO reacts with O in the upper stratosphere, or when ClO reacts with itself in the chemistry of the Antarctic ozone hole.

Paul J. Crutzen, Mario J. Molina and F. Sherwood Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work describing the formation and decomposition of stratospheric ozone.

Aircraft flight

Commercial airliners typically cruise at altitudes of 9–12 km (30,000–39,000 ft) which is in the lower reaches of the stratosphere in temperate latitudes. This optimizes fuel efficiency, mostly due to the low temperatures encountered near the tropopause and low air density, reducing parasitic drag on the airframe. Stated another way, it allows the airliner to fly faster while maintaining lift equal to the weight of the plane. (The fuel consumption depends on the drag, which is related to the lift by the lift-to-drag ratio.) It also allows the airplane to stay above the turbulent weather of the troposphere.

The Concorde aircraft cruised at Mach 2 at about 19,000 m (62,000 ft), and the SR-71 cruised at Mach 3 at 26,000 m (85,000 ft), all within the stratosphere.

Because the temperature in the tropopause and lower stratosphere is largely constant with increasing altitude, very little convection and its resultant turbulence occurs there. Most turbulence at this altitude is caused by variations in the jet stream and other local wind shears, although areas of significant convective activity (thunderstorms) in the troposphere below may produce turbulence as a result of convective overshoot.

On October 24, 2014, Alan Eustace became the record holder for reaching the altitude record for a manned balloon at 135,890 ft (41,419 m). Eustace also broke the world records for vertical speed skydiving, reached with a peak velocity of 1,321 km/h (822 mph) and total freefall distance of 123,414 ft (37,617 m) – lasting four minutes and 27 seconds.

Circulation and mixing

The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which the horizontal mixing of gaseous components proceeds much more rapidly than does vertical mixing. The overall circulation of the stratosphere is termed as Brewer-Dobson circulation, which is a single celled circulation, spanning from the tropics up to the poles, consisting of the tropical upwelling of air from the tropical troposphere and the extra-tropical downwelling of air. Stratospheric circulation is a predominantly wave-driven circulation in that the tropical upwelling is induced by the wave force by the westward propagating Rossby waves, in a phenomenon called Rossby-wave pumping.

An interesting feature of stratospheric circulation is the quasi-biennial oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers, such as ozone or water vapor.

Another large-scale feature that significantly influences stratospheric circulation is the breaking planetary waves resulting in intense quasi-horizontal mixing in the midlatitudes. This breaking is much more pronounced in the winter hemisphere where this region is called the surf zone. This breaking is caused due to a highly non-linear interaction between the vertically propagating planetary waves and the isolated high potential vorticity region known as the polar vortex. The resultant breaking causes large scale mixing of air and other trace gases throughout the midlatitude surf zone. The timescale of this rapid mixing is much smaller than the much slower timescales of upwelling in the tropics and downwelling in the extratropics.

During northern hemispheric winters, sudden stratospheric warmings, caused by the absorption of Rossby waves in the stratosphere, can be observed in approximately half of winters when easterly winds develop in the stratosphere. These events often precede unusual winter weather and may even be responsible for the cold European winters of the 1960s.

Stratospheric warming of the polar vortex results in its weakening. When the vortex is strong, it keeps the cold, high pressure air masses contained in the Arctic; when the vortex weakens, air masses move equatorward, and results in rapid changes of weather in the mid latitudes.

Life

Bacteria

Bacterial life survives in the stratosphere, making it a part of the biosphere. In 2001, dust was collected at a height of 41 kilometres in a high-altitude balloon experiment and was found to contain bacterial material when examined later in the laboratory.

Birds

Some bird species have been reported to fly at the upper levels of the troposphere. On November 29, 1973, a Rüppell's vulture (Gyps rueppelli) was ingested into a jet engine 11,278 m (37,000 ft) above the Ivory Coast, and bar-headed geese (Anser indicus) reportedly overfly Mount Everest's summit, which is 8,848 m (29,029 ft).

Discovery

In 1902, Léon Teisserenc de Bort from France and Richard Assmann from Germany, in separate but coordinated publications and following years of observations, published the discovery of an isothermal layer at around 11–14 km, which is the base of the lower stratosphere. This was based on temperature profiles from mostly unmanned and a few manned instrumented balloons.

Jai Ganesh · 2021-03-22 00:12:43

945) Ionosphere

The ionosphere is the ionized part of Earth's upper atmosphere, from about 48 km (30 mi) to 965 km (600 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.

Ionosphere and magnetosphere

Ionosphere and magnetosphere, regions of Earth’s atmosphere in which the number of electrically charged particles—ions and electrons—are large enough to affect the propagation of radio waves. The charged particles are created by the action of extraterrestrial radiation (mainly from the Sun) on neutral atoms and molecules of air. The ionosphere begins at a height of about 50 km (30 miles) above the surface, but it is most distinct and important above 80 km (50 miles). In the upper regions of the ionosphere, beginning several hundred kilometres above Earth’s surface and extending tens of thousands of kilometres into space, is the magnetosphere, a region where the behaviour of charged particles is strongly affected by the magnetic fields of Earth and the Sun. It is in the lower part of the magnetosphere that overlaps with the ionosphere that the spectacular displays of the aurora borealis and aurora australis take place. The magnetosphere also contains the Van Allen radiation belts, where highly energized protons and electrons travel back and forth between the poles of Earth’s magnetic field.

Ionosphere

Discovery of the ionosphere

Discovery of the ionosphere extended over nearly a century. As early as 1839, the German mathematician Carl Friedrich Gauss speculated that an electrically conducting region of the atmosphere could account for observed variations of Earth’s magnetic field. The notion of a conducting region was reinvoked by others, notably in 1902 by the American engineer Arthur E. Kennelly and the English physicist Oliver Heaviside, to explain the transmission of radio signals around the curve of Earth’s surface before definitive evidence was obtained in 1925. For some years the ion-rich region was referred to as the Kennelly-Heaviside layer.

The name “ionosphere” was introduced first in the 1920s and was formally defined in 1950 by a committee of the Institute of Radio Engineers as “the part of the earth’s upper atmosphere where ions and electrons are present in quantities sufficient to affect the propagation of radio waves.” Much of the early research on the ionosphere was carried out by radio engineers and was stimulated by the need to define the factors influencing long-range radio communication. Subsequent research has focused on understanding the ionosphere as the environment for Earth-orbiting satellites and, in the military arena, for ballistic missile flight. Scientific knowledge of the ionosphere has grown tremendously, fueled by a steady stream of data from spacecraft-borne instruments and enhanced by measurements of relevant atomic and molecular processes in the laboratory.

The ionosphere is the part of the atmosphere that is ionized by solar radiation.

It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere.

It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.

The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles).

The atmosphere above 10 km is called the stratosphere, followed by the mesosphere.

It is in the stratosphere that incoming solar radiation creates the ozone layer.

At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion.

The number of these free electrons is sufficient to affect radio propagation.

This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere.

In a plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.

The ionosphere has irregular patches of ionization.

Jai Ganesh · 2021-03-23 00:14:16

946) Exosphere

Exosphere, outermost region of a planet’s atmosphere, where molecular densities are low and the probability of collisions between molecules is very small. The base of the exosphere is called the critical level of escape because, in the absence of collisions, lighter, faster-moving atoms such as hydrogen and helium may attain velocities that allow them to escape the planet’s gravitational field. Most molecules, however, have velocities considerably lower than the escape velocity, so their rate of escape to outer space is quite low.

The Earth’s exosphere begins about 500 km (300 miles) above the terrestrial surface and extends out through the magnetosphere and beyond to the interplanetary medium. Temperatures in the Earth’s exosphere remain constant with altitude, averaging about 1500 K. The Earth’s exosphere contains the hydrogen geocorona and the Van Allen radiation belts.

The exosphere is the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. The air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space.

The layer directly below the exosphere is the thermosphere; the boundary between the two is called the thermopause. The bottom of the exosphere is sometimes also referred to as the exobase. The altitude of the lower boundary of the exosphere varies. When the Sun is active around the peak of the sunspot cycle, X-rays and ultraviolet radiation from the Sun heat and "puff up" the thermosphere - raising the altitude of the thermopause to heights around 1,000 km (620 miles) above Earth's surface. When the Sun is less active during the low point of the sunspot cycle, solar radiation is less intense and the thermopause recedes to within about 500 km (310 miles) of Earth's surface.

Not all scientists agree that the exosphere is really a part of the atmosphere. Some scientists consider the thermosphere the uppermost part of Earth's atmosphere, and think that the exosphere is really just part of space. However, other scientists do consider the exosphere part of our planet's atmosphere.

Since the exosphere gradually fades into outer space, there is no clear upper boundary of this layer. One definition of the outermost limit of the exosphere places the uppermost edge of Earth's atmosphere around 190,000 km (120,000 miles), about halfway to the Moon. At this distance, radiation pressure from sunlight exerts more force on hydrogen atoms than does the pull of Earth's gravity. A faint glow of ultraviolet radiation scattered by hydrogen atoms in the uppermost atmosphere has been detected at heights of 100,000 km (62,000 miles) by satellites. This region of UV glow is called the geocorona.

Below the exosphere, molecules and atoms of atmospheric gases constantly collide with each other. However, the air in the exosphere is so thin that such collisions are very rare. Gas atoms and molecules in the exosphere move along "ballistic trajectories", reminiscent of the arcing flight of a thrown ball (or shot cannonball!) as it gradually curves back towards Earth under the pull of gravity. Most gas particles in the exosphere zoom along curved paths without ever hitting another atom or molecule, eventually arcing back down into the lower atmosphere due to the pull of gravity. However, some of the faster-moving particles don't return to Earth - they fly off into space instead! A small portion of our atmosphere "leaks" away into space each year in this way.

Although the exosphere is technically part of Earth's atmosphere, in many ways it is part of outer space. Many satellites, including the International Space Station (ISS), orbit within the exosphere or below. For example, the average altitude of the ISS is about 330 km (205 miles), placing it in the thermosphere below the exosphere! Although the atmosphere is very, very thin in the thermosphere and exosphere, there is still enough air to cause a slight amount of drag force on satellites that orbit within these layers. This drag force gradually slows the spacecraft in their orbits, so that they eventually would fall out of orbit and burn up as they re-entered the atmosphere unless something is done to boost them back upwards. The ISS loses about 2 km (1.2 miles) in altitude each month to such "orbital decay", and must periodically be given an upward boost by rocket engines to keep it in orbit.

Jai Ganesh · 2021-03-24 00:16:15

947) Magnetosphere

Magnetosphere, region in the atmosphere where magnetic phenomena and the high atmospheric conductivity caused by ionization are important in determining the behaviour of charged particles.

The Earth, in contrast to Mars and Venus, has a significant surface magnetic field (approximately 0.5 gauss), which, like its gravitational field, becomes weaker as the distance from the centre of the Earth increases. In the direction of the Sun, at approximately 10 Earth radii (almost 65,000 km, or 40,000 miles), the magnetic field is so weak that the pressure associated with particles escaping from the Earth’s gravity is comparable to the opposing pressure associated with the solar wind—the flux mainly of protons and electrons escaping from the Sun’s gravitational field. This equilibrium region, with a characteristic thickness of 100 km (60 miles), is called the magnetopause and marks the outer boundary of the magnetosphere. The lower boundary of the magnetosphere is several hundred kilometres above the Earth’s surface.

On the night side, or the side away from the Sun, the forces associated with the magnetic field and the solar wind are parallel, and thus the magnetosphere extends a considerable distance, possibly even several astronomical units (one astronomical unit is the average distance between the Earth and the Sun, about 1.5 × {10}^8 km).

In the direction perpendicular to the solar wind, the random motion of the solar-wind particles exerts a small pressure on the magnetic field, constricting the magnetosphere slightly. The net result is that the shape of the magnetosphere in gross terms is similar to that of a comet, with the Earth located near the nucleus, or head, of the “comet” and the magnetospheric “tail” trailing out well beyond the Earth, away from the Sun.

Between 10 and 13 Earth radii toward the Sun, there exists the magnetosheath, a region of magnetic turbulence in which both the magnitude and direction of Earth’s magnetic field vary erratically. This disturbed region is thought to be caused by the production of magnetohydrodynamic shock waves, which in turn are caused by high-velocity solar-wind particles. Ahead of this bow shock boundary, toward the Sun, is the undisturbed solar wind.

Jai Ganesh · 2021-03-25 00:10:34

948) Watch

Watch, portable timepiece that has a movement driven either by spring or by electricity and that is designed to be worn or carried in the pocket.

Mechanical Watches

The first watches appeared shortly after 1500, early examples being made by Peter Henlein, a locksmith in Nürnberg, Ger. The escapement used in the early watches was the same as that used in the early clocks, the verge. Early watches were made notably in Germany and at Blois in France, among other countries, and were generally carried in the hand or worn on a chain around the neck. They usually had only one hand for the hours.

The mainspring, the element that drives the watch, consists of a flat spring-steel band stressed in bending or coiling; when the watch, or other spring-driven mechanism, is wound, the curvature of the spring is increased, and energy is thus stored. This energy is transmitted to the oscillating section of the watch (called the balance) by the wheeltrain and escapement, the motion of the balance itself controlling the release of the escapement and consequently the timing of the watch. A friction drive permits the hand to be set.

One of the main defects of the early watches was the variation in the torque exerted by the mainspring; that is, the force of the mainspring was greater when fully wound than when it was almost run down. Since the timekeeping of a watch fitted with a verge escapement was greatly influenced by the force driving it, this problem was quite serious. Solution of the problem was advanced almost as soon as the mainspring was invented (about 1450) by the application of the fusee, a cone-shaped, grooved pulley used together with a barrel containing the mainspring. With this arrangement, the mainspring was made to rotate a barrel in which it was housed; a length of catgut, later replaced by a chain, was wound on it, the other end being coiled around the fusee. When the mainspring was fully wound, the gut or chain pulled on the smallest radius of the cone-shaped fusee; as the mainspring ran down, the leverage was progressively increased as the gut or chain pulled on a larger radius. With correct proportioning of mainspring and fusee radii, an almost constant torque was maintained as the mainspring unwound.

The going barrel, in which the mainspring barrel drives the wheeltrain directly, is fitted to all modern mechanical watches and has superseded the fusee. With better quality mainsprings, torque variations have been reduced to a minimum, and with a properly adjusted balance and balance spring, good timekeeping is ensured.

Up to about 1580, the mechanisms of German watches were made almost wholly of iron; about this time, brass was introduced.

In the earliest watches a plain wheel, known as the balance, was used to control the rate of going of the mechanism. It was subjected to no consistent restoring force; consequently, its period of oscillation and, hence, the rate of the timekeeper were dependent on the driving force. This explains the great importance of the fusee.

Controlling the oscillations of a balance with a spring was an important step in the history of timekeeping. English physicist Robert Hooke designed a watch with a balance spring in the late 1650s; there appears to be no evidence, however, that the spring was in the form of a spiral, a crucial element that would become widely employed. Dutch scientist Christiaan Huygens was probably the first to design (1674–75) a watch with a spiral balance spring. The balance spring is a delicate ribbon of steel or other suitable spring material, generally wound into a spiral form. The inner end is pinned into a collet (a small collar), which fits friction-tight on the balance staff, while the outer end is held in a stud fixed to the movement. This spring acts on the balance as gravity does on the pendulum. If the balance is displaced to one side, the spring is wound and energy stored in it; this energy is then restored to the balance, causing it to swing nearly the same distance to the other side if the balance is released.

If there were no frictional losses (e.g., air friction, internal friction in the spring material, and friction at the pivots), the balance would swing precisely the same distance to the other side and continue to oscillate indefinitely; because of these losses, however, the oscillations in practice die away. It is the energy stored in the mainspring and fed to the balance through the wheel train and escapement that maintains the oscillations.

The performance of the modern watch depends on the uniformity of the period of oscillation of the balance—i.e., the regularity of its movement. The balance takes the form of a wheel with a heavy rim, while the spring coupled to it provides the restoring torque. The balance possesses inertia, dependent on its mass and configuration. The spring should ideally provide a restoring force directly proportional to the displacement from its unstressed or zero position.

The balance is mounted on a staff with pivots, and, in watches of good quality, these run in jewels. Two jewels are used at each end of the balance staff, one pierced to provide a bearing, the other a flat end stone providing axial location by bearing against the domed end of the pivot. Frictional effects at the pivots influence the performance of the watch in various positions—for example, lying and hanging.

The balance and spring can be brought to time, or “regulated,” by varying either the restoring couple provided by the spring or the moment of inertia of the balance. In the first case (by far the more common), this is generally effected by providing a pair of curb pins mounted on a movable regulator index that lengthen or shorten the balance spring as needed.

In the second instance, screws are provided at opposite points on the rim of the balance; these screws are friction-tight in their holes and thus can be moved in or out so as to adjust the inertia of the balance. In “free-sprung” watches no regulator index is provided, and the only adjusters are the screws on the balance rim.

Many modern mechanical watches use a lever escapement, invented in England about 1755 by Thomas Mudge, that leaves the balance free to oscillate, coupling to it only while delivering the impulse, taken from the mainspring via the wheel train and while being unlocked by the balance. It was developed into its modern form with the club-tooth escape wheel at the beginning of the 19th century but was not universally adopted until the early 20th century. In good-quality watches the club-tooth escape wheel is made of hardened steel, with the acting surfaces ground and polished. An improved form of the lever escapement is characterized by a double-roller safety action in which the intersection between the guard pin and roller, which takes place underneath the roller, is much deeper than in early single-roller watches; thus, any friction caused by jolts encountered in wear causes less constraint on the balance and less endangerment of the timekeeping properties of the watch. By far the most important watch escapement today is the lever escapement; it is used in its jeweled form in watches of moderate to excellent quality, and it is used with steel pallet pins and a simplified fork-and-roller action in cheaper watches (known as pin-pallet watches).

In the wheel train of a modern watch, it is necessary to achieve a step-up ratio of approximately 1 to 4,000 between barrel and escape wheel. This involves four pairs of gears, the ratio per pair commonly being between 6 to 1 and 10 to 1. Because of space considerations, the pinions must have a low number of leaves (teeth), commonly 6 to 12. This entails a number of special gearing problems, aggravated by the fineness of the pitch. Any error in centre distance, form, or concentricity is therefore proportionately more important than in larger gear trains.

The first patent covering the application of jewels in watches was taken out in London in 1704; diamonds and sapphires were used. Synthetic jewels made from fused powdered alumina (aluminum oxide) are now commonly used. Watch jewels are given a very high polish; a uniform outside diameter for the jewel bearings is highly important, because they are pressed into accurately sized holes smaller than the jewels themselves and held there by friction.

The first patent on the self-winding pocket watch was taken out in London in 1780. An English invention patented in 1924, the self-winding wristwatch by Louis Recordon, contains a swinging weight pivoted at the centre of the movement, coupled to the barrel arbor through reduction wheels and gears. A more modern self-winding watch is fitted with a weight or rotor swinging 360 degrees and winding in both directions.

Electric-Powered And Electronic Watches

Electric-powered watches use one of three drive systems: (1) the galvanometer drive, consisting of the conventional balance-hairspring oscillator, kept in motion by the magnetic interaction of a coil and a permanent magnet, (2) the induction drive, in which an electromagnet attracts a balance containing soft magnetic material, or (3) the resonance drive, in which a tiny tuning fork (about 25 mm [1 inch] in length), driven electrically, provides the motive power. Both galvanometer and induction drive types use a mechanical contact, actuated by the balance motion, to provide properly timed electric-drive pulses. Each oscillation of the balance operates a time-indicating gear train by advancing a toothed wheel one tooth. First produced in 1953, the resonance drive type, properly called an electronic watch, is inherently more accurate since it operates at a frequency higher than that customarily used with balance-type watches, and the tuning fork is a fairly stable source of frequency. The higher frequency requires the replacement of a mechanical contact by a transistor. The minute and rapid motion of the tuning fork moves forward an extremely fine-toothed ratchet wheel. There is very little friction in the electronic watch; only tiny amounts of oil are needed. When the battery is too weak to operate the tuning fork, the watch simply stops, without deterioration. Miniature high-energy-density batteries are used as power sources in all three types.

The progressive miniaturization of electronic components in the late 20th century made possible the development of all-electronic watches, in which the necessary transistors, resistors, capacitors, and other elements were all on one or several miniature integrated circuits, or chips. The complex circuitry of such watches enabled them to perform a variety of timekeeping functions and also made possible digital readouts of the time in place of the traditional second, minute, and hour hands.

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Jai Ganesh · 2021-03-26 00:05:20

949) Smartwatch

Smartwatch, a small smartphonelike device worn on the wrist. Many smartwatches are connected to a smartphone that notifies the user of incoming calls, e-mail messages, and notifications from applications. Some smartwatches can even make telephone calls. Many smartwatches have colour displays, but some inexpensive models use a black-and-white “e-paper” display. The user can operate the smartwatch through a touch screen, physical buttons, or a combination of the two. Some smartwatches come with pedometers and heart-rate monitors to help users track their health.

As computers shrank in size in the 20th century, electronic equipment became small enough to fit into a watch. One of the first calculator watches was the Calcron (1975), which had a nine-digit display. In the early 1980s, Seiko introduced several watches with computing capabilities. The Data-2000 (1983), so called because it could store two memos of up to 1,000 characters each, fit into a keyboard on which the user entered information. The RC-1000 (1984) connected to a personal computer. The Receptor MessageWatch (1990) received pager messages via FM radio signals.

The 1990s saw a further melding of watches and computers. Users of the Datalink (1994), a collaboration between Timex and Microsoft, entered information on their personal computers, such as phone numbers, which were then transmitted to the watch wirelessly, using light pulses. Seiko’s Ruputer (1998) was a personal computer that fit into a watch. Data were entered through buttons or a joystick, and users could write their own software. Samsung’s SPH-WP10 (1999) was the first watch phone; its battery lasted for 90 minutes of calling time.

One of the earliest true smartwatches was the Microsoft SPOT (Smart Personal Object Technology), introduced in 2004. The SPOT received information such as weather, news, and stock updates through FM radio. It also received e-mail and instant messages, but users could not reply. With the rise of the smartphone, smartwatches such as the Sony Ericsson LiveView (2010), the Pebble (2013), and the Apple Watch (2015) emerged that received data from a phone. In 2014 Google developed Android Wear—a version of its mobile operating system, Android—specifically for wearable devices like smartwatches.

Jai Ganesh · 2021-03-27 00:16:43

950) Gymnastics

Gymnastics, the performance of systematic exercises—often with the use of rings, bars, and other apparatus—either as a competitive sport or to improve strength, agility, coordination, and physical conditioning.

History

The term gymnastics, derived from a Greek word meaning “to exercise unaided,” applied in ancient Greece to all exercises practiced in the gymnasium, the place where male athletes did indeed exercise unclothed. Many of these exercises came to be included in the Olympic Games, until the abandonment of the Games in 393 CE. Some of the competitions grouped under this ancient definition of gymnastics later became separate sports such as athletics (track and field), wrestling, and boxing.

Of the modern events currently considered to be gymnastics, only tumbling and a primitive form of vaulting were known in the ancient world. For instance, Egyptian hieroglyphs show variations of backbends and other stunts being performed with a partner, while a well-known fresco from Crete at the palace at Knossos shows a leaper performing what is either a cartwheel or handspring over a charging bull. Tumbling was an art form in ancient China as well. Stone engravings found in Shandong province that date to the Han period (206 BCE–220 CE) portray acrobatics being performed.

Tumbling continued in the Middle Ages in Europe, where it was practiced by traveling troupes of thespians, dancers, acrobats, and jugglers. The activity was first described in the West in a book published in the 15th century by Archange Tuccaro, Trois dialogues du Sr. Archange Tuccaro (the book contains three essays on jumping and tumbling). Tumbling seems to be an activity that evolved in various forms in many cultures with little cross-cultural influence. For instance, the hoop-diving illustrated in Tuccaro’s book looks very similar to a type of tumbling seen in ancient China. Tumbling and acrobatics of all kinds were eventually incorporated into the circus, and it was circus acrobats who first used primitive trampolines.

Jean-Jacques Rousseau’s novel Émile; ou, de l’éducation (1762; Emile; or, On Education) is credited by historians as the catalyst of educational reform in Europe that combined both the physical and cognitive training of children. Rousseau’s work inspired educational reformers in Germany, who opened schools known as Philanthropinum in the late 1700s that featured a wide variety of outdoor activities, including gymnastics; children from all economic strata were accepted. The “grandfather” of modern gymnastics, Johann Christoph Friedrich Guts Muths (1759–1839), was a leading teacher at the Philanthropinist school in Schnepfenthal. In his seminal work, Gymnastik für die Jugend (1793; Gymnastics for Youth), Guts Muths envisioned two main divisions of gymnastics: natural gymnastics and artificial gymnastics. These two divisions may be thought of as utilitarian and nonutilitarian gymnastics. The former disciplines emphasize the health of the body, similar to the exercises developed in Sweden and Denmark under Per Henrik Ling (1776–1839) and Neils Bukh (1880–1950), respectively. Modern aerobics also falls into this category; indeed, sports aerobics has recently been added to the disciplines sponsored by the International Gymnastics Federation. In contrast, nonutilitarian gymnastics is characterized by modern artistic gymnastics, the maneuvers of which are geared to beauty and not function. For example, in feudal Europe young men were taught to mount and dismount a horse, useful knowledge during a time when armies rode. Modern “horse” work in artistic gymnastics has evolved to a point where there is no practical connection between gymnastic maneuvers on a horse and horsemanship. Only the language of riding remains, with the terms “mount” and “dismount” still being used in gymnastics.

The prime developer of natural gymnastics was Per Henrik Ling. In 1813 Ling founded a teacher-training centre, the Royal Gymnastics Central Institute, in Stockholm. Ling devised and taught a system of gymnastic exercises designed to produce medical benefits for the athlete. Calisthenics are attributed to him, including free calisthenics—that is, exercises without the use of hand apparatus such as clubs, wands, and dumbbells. Although Ling did not promote competition, free calisthenics have evolved into the competitive sport now known as floor exercise.

The acknowledged “father” of gymnastics, Friedrich Ludwig Jahn, founder of the Turnverein movement, is credited with the rapid spread of gymnastics throughout the world. Gymnastic competition can be traced to the outdoor playground (Turnplatz) Jahn opened in a field known as the “Hasenheide” (rabbit field) on the outskirts of Berlin. Ernst Eiselen, Jahn’s assistant and the coauthor of Die Deutsche Turnkunst (1816; The German Gymnastic Art), carefully noted and explained the various exercises developed on the playground. The pommel horse was used for leg-swinging exercises and for vaulting. Jahn invented the parallel bars to increase the upper-body strength of his students, and immense towers were erected to test their courage. Balance beams, horizontal bars, climbing ropes, and climbing poles were also found at the Turnplatz. Primitive pole vaulting was practiced along with other athletic games. The wide variety of challenging apparatus found on the playground attracted young men who were then, in addition, indoctrinated with Jahn’s dream of German unification and his ideas on the defense of the fatherland and ridding Prussia of French influence.

The Prussians and leaders from surrounding countries became wary of nationalist sentiments, and Jahn and his followers were viewed with suspicion after the defeat of Napoleon in 1813. By 1815 student organizations such as the Burschenschaft (“Youth Branch”) were in favour of adopting a constitutional form of government, arming the citizenry, and instituting greater civil freedoms. In 1819, after the murder of the German playwright August von Kotzebue by a Burschenschaft gymnast, the Prussian king Frederick William III closed approximately 100 gymnastics fields and centres in Prussia. Other Germanic states followed suit. Jahn was arrested, jailed as a democratic demagogue, and placed under house arrest for the next five years. He was eventually acquitted but was admonished to relocate far from Berlin to a city or town with neither institutions of higher learning or gymnasia. He was awarded a yearly stipend and settled in Freiburg. The time was a period of personal tragedy for Jahn; two of his three children died while he was under house arrest, and his wife died shortly thereafter. Three of his close followers, Karl Beck, Karl Follen, and Franz Lieber, fearing arrest, fled to North America, bringing gymnastics with them. The Turners remaining in Prussia went underground until the ban on gymnastics was lifted by King Frederick William IV in 1842.

The first German gymnastic festival (Turnfest) was held in Coburg in 1860. The festival attracted affiliated Turnverein clubs and marked the beginning of international competition, as the growing family of Turners outside of Germany were invited to participate. Americans had been introduced to gymnastics by followers of Jahn in the late 1820s, but not until 1848, when large numbers of Germans immigrated, did transplanted Turnverein members organize clubs and establish a national union of Turner societies. (A similar movement, the Sokol, originated and spread in Bohemia and was also transported to the United States.) By 1861 American Turners and Turners from Germanic regions bordering Prussia attended the second Turnfest in Berlin. By the time of the first modern Olympics in Athens in 1896, eight Turnfests had taken place in Germany with the participation of a growing number of countries.

In 1881 the Fédération Internationale Gymnastique (FIG) was founded to supervise international competition. The 1896 Olympic Games fostered interest in gymnastics, and the FIG World Championships in gymnastics were organized for men in 1903, for women in 1934.

The 1896 Olympic Games marked the advent of true international, open competition in gymnastics. The Games featured typical German, or “heavy apparatus,” events and rope climbing. Gymnastics competitions were not standardized nor free of track-and-field events until the 1928 Olympics, when five of the six events presently held in Olympic gymnastics were contested—pommel horse, rings, vaulting, parallel bars, and horizontal bar, with both compulsory and optional routines required. Women first competed in the Olympics in 1928 in events similar to those of the men except for the addition of the balance beam. Floor exercise events were added in 1932.

Gymnasts

Many of the world’s greatest gymnasts have come from eastern Europe. Larisa Latynina of Ukraine, later the coach of the Soviet Union team, is widely considered the greatest female gymnast of all time; she was the all-around champion in two Olympics (1956 and 1960) and two world championships (1958 and 1962). No other gymnast has achieved this distinction. Latynina’s prime rival was Věra Čáslavská of Czechoslovakia, who later became the Czech Republic’s Minister of Sport. Čáslavská was all-around champion three times, including two Olympics (1964 and 1968) and one world championship (1966).

In the 1970s a major change had occurred in women’s gymnastics as younger and younger girls began competing in events. Russian gymnast Olga Korbut and the Romanian Nadia Comăneci were both young teens during their Olympic triumphs. The presence of a preponderance of teenage girls in international gymnastics competition from the late 1970s and into the 21st century was directly related to the Korbut-Comăneci phenomenon. Many of these younger gymnasts, especially those who trained long hours for competitions, had not yet reached menarche, and some used doping techniques to delay the onset of physical maturation and its resulting changes to a gymnast’s centre of gravity and weight. Coaching these youngsters posed difficulties since many were lured from or pushed by their families to train in unfamiliar surroundings. By 2000 the age requirement for Olympic participants in gymnastics had been raised to 16 to offset some of these problems.

In men’s gymnastics the greatest champions were Viktor Chukarin of the Soviet team and Katō Sawao of Japan—each two-time Olympic all-around champions (Chukarin in 1952 and 1956, Sawao in 1968 and 1972)—along with Vitaly Scherbo of Belarus, an Olympic (1992) and world (1993) all-around champion.

Olympic gymnastics are grouped into different divisions—artistic, rhythmic, and trampoline. For men the artistic gymnastics events are: floor exercise, pommel horse, rings, vault, parallel bars, horizontal bar, and combined exercises (the all-around), which combines the scores of the other six events. The combined exercises for men are contested both on an individual and on a team basis. For women the artistic events are floor exercise, vault, uneven bars, balance beam, and combined exercises, both team and individual.

Rhythmic group gymnastics was originally required in the women’s artistic program but became a separate sport when it was introduced internationally at an invitational competition in Budapest, Hungary, in 1963. Thereafter the Fédération Internationale Gymnastique (FIG) scheduled a world competition in the even-numbered years beginning in 1964. First known as modern rhythmic gymnastics, and later as rhythmic sport gymnastics, the discipline now known as rhythmic gymnastics became an Olympic sport in 1984. This branch of gymnastics is practiced only by women. The events in rhythmic gymnastics are named for the hand apparatus employed by the gymnast: rope, hoop, ball, clubs, and ribbon. Medals are awarded at the Olympics and world championships for team, group, all-around, and individual event competition.

Trampoline and tumbling are also under the aegis of the FIG. Trampoline debuted as a men’s and women’s event at the 2000 Olympic Games; Olympic competition is individual only. World championship trampoline events also include double mini-trampoline and synchronized trampoline competition. In the latter, two gymnasts perform the same routine on two trampolines placed side by side.

Sports acrobatics has been contested internationally since 1973. In 1998 the International Federation of Sports Acrobatics voted to dissolve and the sport was subsumed by the FIG. The events in sports acrobatics are: women’s pairs, mixed pairs, men’s pairs, women’s trios, and men’s fours. Pairs and group exercises are performed to a musical accompaniment on a free-exercise-type platform. There are several routines, some of which must include “human pyramids” that are created by the gymnasts and must be held for four seconds to be scored; the pairs exercise must contain at least six partner-balance elements held for two seconds; and throws with twisting and somersaulting interspersed with tumbling elements must also be included.

The final discipline sanctioned by the FIG is sports aerobics. Aerobics exercise has been a popular form of physical training for the general public since the mid-1970s. The highly competitive sports version of aerobics features routines of less than two minutes’ duration performed by individual men, mixed pairs, individual women, and trios. The sport was first found in the program of general gymnastics in the late 1980s. In 1994 the FIG congress decided to organize the World Aerobic Championships and to structure sports aerobics similarly to its other competitive disciplines. The first official world championships were held in 1995 in Paris with 34 countries participating. In 1997 the International World Games Association included sports aerobics in the fifth World Games. Sports acrobatics and sports aerobics have not yet attained Olympic status.

Jai Ganesh · 2021-03-28 00:02:52

951) Stroke

Stroke, also called apoplexy, sudden impairment of brain function resulting either from a substantial reduction in blood flow to some part of the brain or from intracranial bleeding. The consequences of stroke may include transient or lasting paralysis on one or both sides of the body, difficulties in speaking or eating, and a loss of muscular coordination. A stroke may cause cerebral infarctions—dead sections of brain tissue.

Risk Factors

Aging is one of the greatest risk factors for stroke, with the risk increasing significantly after age 55. Other major risk factors include hypertension (high blood pressure), smoking, high cholesterol, diabetes, obesity, heart disease, excessive alcohol consumption, physical inactivity, and heritable defects. Chronic stress, which increases cortisol levels that in turn lead to increases in blood pressure and blood cholesterol, can also raise the risk of stroke. In addition, stroke is more common in men than in women, and the risk of death from stroke is higher in African Americans than in Caucasians. This difference is primarily due to the higher prevalence of hypertension in African Americans.

Some people are predisposed to stroke because of genetic factors. For example, some persons with the inherited blood disorder sickle cell anemia have a significantly increased risk of stroke. Certain genetic variations not otherwise known to be associated with inherited disease also have been associated with risk. For example, variations on chromosomes 1, 6, and 19 have been related to risk of intracerebral hemorrhage.

Causes

The most common cause of stroke is a blood clot (thrombus) that has formed within a blood vessel of the brain. A blood clot also can lodge in an artery supplying brain tissue after originating in another portion of the body and traveling to the brain. This is known as an embolism. A myocardial infarction (heart attack), damage to a heart valve, and an irregular heartbeat called atrial fibrillation can cause blood clots that may reach the brain. Both types of clots reduce or stop the flow of blood to brain cells. During the early stages of stroke from a blood clot, the condition may be further complicated by the leakage of blood and fluid into the surrounding areas (edema).

Stroke can also occur as a result of atherosclerosis, the buildup of fatty deposits on artery walls. Atherosclerosis can cause intermittent insufficiency in the flow of blood due to spasm of the arteries, which can rupture, or the sludging of the blood as it passes through segments of vessels that have been narrowed by fatty deposits.

Types And Symptoms

A hemorrhagic stroke, involving intracranial bleeding, may occur after an artery ruptures, usually as a result of a weakening of the arterial wall because of atherosclerosis or because of a thinning of the wall along with bulging (aneurysm), often due to hypertension.

Transient ischemic attacks, or mini-strokes, result when long, thin arteries penetrating deep into the brain become blocked by atherosclerosis, causing areas of surrounding tissue to lose their blood supply. The tissue may then wither, creating minute holes, called lacunes. A succession of transient ischemic attacks over the years can riddle the brain, causing dementia.

The initial onset of stroke may be massive in its effects, producing widespread paralysis, inability to speak, coma, or death within a short time, usually within several hours or days. On the other hand, the onset may be manifested by a series of transient ischemic attacks during which the patient may experience weakness and numbness of an arm, a leg, or a side of the face. There may be temporary difficulty in speech, confusion, or visual disturbances. Transient ischemic attacks may recur many times, but they are usually followed eventually by more widespread and permanent paralysis.

The groups of muscles and nerves involved are a direct reflection of the artery and brain tissues involved. If the left side of the brain (the dominant side for most persons) is affected, there is a paralysis of the right side of the body because most of the nerves cross to the opposite side of the body from their origin in the brain. However, the combinations of signs and symptoms are innumerable.

Diagnosis And Treatment

Precise history and physical examination, especially for neurological changes, are essential to differentiate stroke from a tumour and from brain injury resulting from other causes. Differential diagnosis is carried out by examining the spinal fluid for evidence of blood and by performing diagnostic imaging (as by computed tomography [CT] scan).

A critical step in the diagnostic process is to determine whether the stroke is due to a thrombus, an embolism, or a hemorrhage. Anticoagulant drugs, such as tissue plasminogen activator (e.g., alteplase or reteplase), are widely used in the treatment of stroke due to thromboses or emboli; however, such drugs are contraindicated when the stroke is due to hemorrhage. A stroke may have both a clotting and a significant hemorrhagic factor present, which can present further difficulties. Many strokes are due to closure of one of the two carotid arteries that supply the brain after passing up the sides of the neck from the aorta. If the closure involves only a small segment, surgery may be attempted to remove the obstruction or to insert a graft or synthetic bypass.

Many persons who have a stroke live for years after the event. Early and persistent efforts for rehabilitation are essential, including physical, occupational, and speech therapy. These therapies typically begin within a day or two after the stroke. Several pharmacological agents and other treatment strategies to enhance the recovery of brain and motor function in stroke patients have been investigated. For example, clinical trials testing the antidepressant Prozac (fluoxetine) in combination with physiotherapy have indicated that the drug can enhance the recovery of motor function in some patients. Cortical stimulation, in which the area of the brain responsible for motor control is stimulated by electrical pulses sent from an implanted device, has met with mixed success in severely compromised stroke patients.

Jai Ganesh · 2021-03-29 00:06:09

952) Electroencephalography

Electroencephalography, technique for recording and interpreting the electrical activity of the brain. The nerve cells of the brain generate electrical impulses that fluctuate rhythmically in distinct patterns. In 1929 German scientist Hans Berger published the results of the first study to employ an electroencephalograph, an instrument that measures and records these brain-wave patterns. The recording produced by such an instrument is called an electroencephalogram, commonly abbreviated EEG.

To record the electrical activity of the brain, 8 to 16 pairs of electrodes are attached to the scalp. Each pair of electrodes transmits a signal to one of several recording channels of the electroencephalograph. This signal consists of the difference in the voltage between the pair. The rhythmic fluctuation of this potential difference is shown as peaks and troughs on a line graph by the recording channel. The EEG of a normal adult in a fully conscious but relaxed state is made up of regularly recurring oscillating waves known as alpha waves. When a person is excited or startled, the alpha waves are replaced by low-voltage rapid irregular waves. During sleep, the brain waves become extremely slow. Such is also the case when a person is in a deep coma. Other abnormal conditions are associated with particular EEG patterns. For example, irregular slow waves known as delta waves arise from the vicinity of a localized area of brain damage.

Electroencephalography provides a means of studying how the brain works and of tracing connections between one part of the central nervous system and another. However, its effectiveness as a research tool is limited, because it records only a small sample of electrical activity from the surface of the brain. Many of the more complex functions of the brain, such as those that underlie emotions and thought, cannot be related closely to EEG patterns. Furthermore, the EEG is of no use in diagnosing psychiatric illness.

Electroencephalography has proved more useful as a diagnostic aid in cases of serious head injuries, brain tumours, cerebral infections, sleep disorders, epilepsy, and various degenerative diseases of the nervous system. Electroencephalography is also useful in the assessment of patients with suspected brain death. This is particularly important if organs are to be saved for transplantation as soon as brain death has been confirmed. Sleep deprivation and other provocative tests, including photic (light) stimulation and hyperventilation, can be used to accentuate borderline findings.

Jai Ganesh · 2021-03-30 00:15:32

953) Epilepsy

Epilepsy, chronic neurological disorder characterized by sudden and recurrent seizures which are caused by an absence or excess of signaling of nerve cells in the brain. Seizures may include convulsions, lapses of consciousness, strange movements or sensations in parts of the body, odd behaviours, and emotional disturbances. Epileptic seizures typically last one to two minutes but can be followed by weakness, confusion, or unresponsiveness. Epilepsy is a relatively common disorder affecting about 40 million to 50 million people worldwide; it is slightly more common in males than females. Causes of the disorder include brain defects, head trauma, infectious diseases, stroke, brain tumours, or genetic or developmental abnormalities. Several types of epileptic disorders are hereditary. Cysticercosis, a parasitic infection of the brain, is a common cause of epilepsy in the developing world. About half of epileptic seizures have an unknown cause and are called idiopathic.

In 1981 the International League Against Epilepsy developed a classification scheme for seizures based on their mode of onset. This work resulted in the formation of two major classes: partial-onset seizures and generalized-onset seizures.

Partial-Onset Seizures

A partial seizure originates in a specific area of the brain. Partial seizures consist of abnormal sensations or movements, and a lapse of consciousness may occur. Epileptic individuals with partial seizures may experience unusual sensations called auras that precede the onset of a seizure. Auras may include unpleasant odours or tastes, the sensation that unfamiliar surroundings seem familiar (déjà vu), and visual or auditory hallucinations that last from a fraction of a second to a few seconds. The individual may also experience intense fear, abdominal pain or discomfort, or an awareness of increased respiration rate or heartbeat. The form of the onset of a seizure is, in most cases, the same from attack to attack. After experiencing the aura, the individual becomes unresponsive but may examine objects closely or walk around.

Jacksonian seizures are partial seizures that begin in one part of the body such as the side of the face, the toes on one foot, or the fingers on one hand. The jerking movements then spread to other muscles on the same side of the body. This type of seizure is associated with a lesion or defect in the area of the cerebral cortex that controls voluntary movement.

Complex partial seizures, also called psychomotor seizures, are characterized by a clouding of consciousness and by strange, repetitious movements called automatisms. On recovery from the seizure, which usually lasts from one to three minutes, the individual has no memory of the attack, except for the aura. Occasionally, frequent mild complex partial seizures may merge into a prolonged period of confusion, which can last for hours or days with fluctuating levels of awareness and strange behaviour. Complex partial attacks may be caused by lesions in the frontal lobe or the temporal lobe.

Studies of temporal lobe epilepsy have provided important insight into the neurological overactivity that is frequently associated with seizures. For example, defects in neurons have long been suspected to underlie most forms of epilepsy characterized by excess brain activity. However, investigations of temporal lobe epilepsy have revealed that abnormal swelling of neuronal support cells known as astrocytes, which serve important functions in regulating neuron activity, may actually give rise to this form of seizure. As a result, astrocyte abnormalities have become of significant interest in understanding the pathology of other forms of epilepsy as well as other types of neurological disease.

Generalized-Onset Seizures

Generalized seizures are the result of abnormal electrical activity in most or all of the brain. This type of seizure is characterized by convulsions, short absences of consciousness, generalized muscle jerks (clonic seizures), and loss of muscle tone (tonic seizures), with falling.

Generalized tonic-clonic seizures, sometimes referred to by the older term grand mal, are commonly known as convulsions. A person undergoing a convulsion loses consciousness and falls to the ground. The fall is sometimes preceded by a shrill scream caused by forcible expiration of air as the respiratory and laryngeal muscles suddenly contract. After the fall, the body stiffens because of generalized tonic contraction of the muscles; the lower limbs are usually extended and the upper limbs flexed. During the tonic phase, which lasts less than a minute, respiration stops because of sustained contraction of the respiratory muscles. Following the tonic stage, clonic (jerking) movements occur in the arms and legs. The tongue may be bitten during involuntary contraction of the jaw muscles, and urinary incontinence may occur. Usually, the entire generalized tonic-clonic seizure is over in less than five minutes. Immediately afterward, the individual is usually confused and sleepy and may have a headache but will not remember the seizure.

Studies measuring electric currents in the heart have demonstrated that some patients affected by tonic-clonic seizures experience abnormal cardiac rhythms either during or immediately after a seizure. In some cases the heart may stop beating for several seconds, a condition known as asystole. Asystole has been linked to a phenomenon called sudden unexpected death in epilepsy (SUDEP), which affects more than 8 percent of epilepsy patients and typically occurs in people between the ages of 20 and 30. The cause of SUDEP is not known with certainty. Scientists suspect that accumulated damage and scarring in cardiac tissue, caused by multiple, recurring seizures, has the potential to interfere with electrical conduction in the heart and thus precipitate SUDEP during a typical tonic-clonic seizure. In addition, genetic defects associated with epilepsy and abnormalities in heart function have been identified in families affected by both inherited epilepsy and SUDEP.

Primary generalized, or absence, epilepsy is characterized by repeated lapses of consciousness that generally last less than 15 seconds each and usually occur many times a day. This type of seizure is sometimes referred to by the older term petit mal. Minor movements such as blinking may be associated with absence seizures. After the short interruption of consciousness, the individual is mentally clear and able to resume previous activity. Absence seizures occur mainly in children and do not appear initially after age 20; they tend to disappear before or during early adulthood. At times absence seizures can be nearly continuous, and the individual may appear to be in a clouded, partially responsive state for minutes or hours.

Diagnosis

A person with recurrent seizures is diagnosed with epilepsy. A complete physical examination, blood tests, and a neurological evaluation may be necessary to identify the cause of the disorder. Electroencephalogram (EEG) monitoring is performed to detect abnormalities in the electrical activity of the brain. Magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), or magnetic resonance spectroscopy (MRS) may be used to locate structural or biochemical brain abnormalities.

Treatment

Most people with epilepsy have seizures that can be controlled with antiepileptic medications such as valproate, ethosuximide, clonazepam, carbamazepine, and primidone; these medications decrease the amount of neuronal activity in the brain. Brain damage caused by epilepsy usually cannot be reversed. Epileptic seizures that cannot be treated with medication may be reduced by surgery that removes the epileptogenic area of the brain. Other treatment strategies include vagus nerve stimulation, a diet high in fat and low in carbohydrates (ketogenic diet), and behavioral therapy. It may be necessary for epileptic individuals to refrain from driving, operating hazardous machinery, or swimming because of the temporary loss of control that occurs without warning.

Family and friends of an epileptic individual should be aware of what to do if a seizure occurs. During a seizure the clothing should be loosened around the neck, the head should be cushioned with a pillow, and any sharp or hard objects should be removed from the area. An object should never be inserted into the person’s mouth during a seizure. After the seizure the head of the individual should be turned to the side to drain secretions from the mouth.

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