In most plants, the root system is a below-ground structure that serves primarily to anchor the plant in the soil and take up water and minerals. Roots may be less familiar than the more visible flowers, stems, and leaves, but they are no less important to the plant. Roots have four regions: a root cap; a zone of division; a zone of elongation; and a zone of maturation. The root cap is a cup-shape…
The rose family (Rosaceae), in the order Rosales, is a large plant family containing more than 100 genera and 2,000 species of trees, shrubs, and herbs. This family is represented on all continents except Antarctica, but the majority of species are found in Europe, Asia, and North America. Fossil evidence from Colorado, reliably identified as belonging to the genus Rosa, suggests that this family …
A rotation is one of three rigid motions that move a figure in a plane without changing its size or shape. As its name implies, a rotation moves a figure by rotating it around a center somewhere on a plane. This center can be somewhere inside or on the figure, or outside the figure completely. The two other rigid motions are reflections and translations. Figure 1 illustrates a rotation of 30 …
With more than 10,000 species described, roundworms (phylum Nematoda) are among the most numerous and widespread animals. They occur in all habitats, including freshwater, marine, and terrestrial ecosystems, from the tropics to the polar regions. They often occur in staggering numbers: 10.8 sq ft (1 sq m) of mud has been found to contain more than four million nematodes. Because of their distribut…
Sleep disorders are chronic sleep irregularities, which drastically interfere with normal nighttime sleep or daytime functioning. Sleep-related problems are the most common complaint heard by doctors and psychiatrists, the two most common being insomnia (inability to go to sleep or stay asleep), and hypersomnia (excessive daytime sleepiness). While most people experience both problems at some time…
Rumination is a specialized digestion process found in most hoofed mammals with an even number of toes-such as cattle, sheep, goats, deer, antelope, camels, buffalo, giraffes, and chevrotains. All of these plant-eating animals lack the enzyme cellulase, which is capable of breaking down the tough cellulose in plant cell walls. The stomach of these grazing herbivores consists of four chambersȁ…
Sleeping sickness is a protozoan infection passed to humans through the bite of the tsetse fly. It progresses to death within months or years if left untreated. Diagnosis of sleeping sickness can be made by microscopic examination of fluid from the site of the tsetse fly bite or swollen lymph nodes for examination. A method to diagnose Rhodesian trypanosome involves culturing blood, bone marrow, o…
A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are the fundamental objects in the universe. They are the factories where elements heavier than hydrogen are formed. The radiation from a typical star like the Sun provides temperate conditions on planets like Earth where life can arise. Since the Sun is obviously the central sourc…
Slime molds are microscopic organisms. As slime molds are eukaryotic organisms, they have their genetic material contained within a membrane inside the cell. Once thought to be fungi, slime molds are now recognized to be very different from fungi. Indeed, slime molds are now classified as one of the five main divisions of life (the other four are fungi, bacteria, plants, and animals). There are th…
Star clusters are groups of stars that occur close to each other in space, appear to have roughly similar ages, and therefore, seem to have had a common origin. Star clusters are typically classified into one of two large subgroups, galactic clusters and globular clusters. Galactic clusters are sometimes also known as open clusters. Astronomers have identified thousands of galactic star clusters i…
Sloths are mammals of the Central and South American jungle that spend their lives in trees, eating leaves in a very slow, or "slothful," manner. They belong to order Edentata, which means "without teeth." However, sloths are not actually without teeth. They have molars, or chewing teeth, that have no roots and continue to grow throughout their lives. Anteaters, for whi…
Rushes are monocotyledonous plants in the genus Juncus. Rushes make up most of the species in the family Juncaceae. There are about 400 species in the rush family, distributed among eight or nine genera. The most species-rich groups are the rushes (Juncus spp.) with 225 species, and the wood-rushes (Luzula spp.) with 80 species. Species in the rush family occur worldwide, but they are particularly…
Slug is a common name for a group of terrestrial snails like molluscs with little or no external shell. Examples of common slugs are Limax maximus, the large garden slug, and Limax agrestis, which eats grain seedlings and is regarded as a farm pest in Europe. Other urban species are Arion circumscriptus and Limax flavus. Slugs are classified in the gastropod subclass Pulmonata. The pulmonates are …
Rusts and smuts are fungi belonging to the orders Urediniales (rusts) and Ustilaginales (smuts) which are basidiomycete fungi. The rusts have complicated life cycles which involve the infection of two different plant species. The most well-known members of these groups are wheat rust (Puccinia graminis tritici) and corn smut (Ustilago myadis). Rust fungi attack plants such as ferns, gymnosperms, a…
The saiga antelope (Saiga tatarica) is a relatively northern, Eurasian antelope in the family Bovidae. Historically, the range of the saiga antelope extended from Poland in the west, to the Caucasus Mountains of northwestern Turkey, Georgia, and Azerbaijan, the vicinity of the Caspian Sea in Kazakhstan, and as far east as Mongolia. However, mostly because of overhunting, this species now only occu…
Star formation is the process by which a cold, dark cloud of gas and dust is transformed into a brilliant star with a surface temperature anywhere from 3,000-50,000K (4,900–90,000°F; 2,700–50,000°C). Many regions of our galaxy are filled with cold clouds of gas that begin to contract, under certain conditions, as a result of their own gravitational attraction. As one of…
Smallpox is an infection caused by the variola virus, a member of the poxvirus family. Throughout history, smallpox has caused huge epidemics resulting in great suffering and enormous death tolls worldwide. In 1980, the World Health Organization (WHO) announced that a massive program of vaccination against the disease had resulted in the complete eradication of the virus (with the exception of sto…
Billions of large, essentially independent groups of stars exist in the universe. These are called galaxies. A galaxy is labeled a starburst galaxy if an exceptionally high rate of star formation is found to be taking place within it. This often occurs in galaxies that are in the process of or have recently undergone a merging or collision with another galaxy. Although astronomers do not know exac…
Smallpox, or variola major, is a highly contagious disease that is caused by the variola virus. The name smallpox comes from the Latin word for spotted. A visual hallmark of smallpox is the raised bumps that appear on the victim's face and body. Smallpox is fatal in approximately 25% of cases. There is no cure for smallpox, and treatment is supportive. Prevention of the disease by the admin…
Starfish are marine invertebrates in the phylum Echinodermata, which also includes sea urchins, brittle stars, sea lilies, and sea cucumbers. Starfish belong to the class Asteroidea, which includes 1,500 species inhabiting the shallow margins of all of the world's oceans. Starfish vary widely in appearance. Some species grow up to 3 ft (1 m) in diameter; others are barely 0.5 in (1.3 cm) ac…
Salamanders and newts are aquatic or amphibious animals in the order Caudata (sometimes known as the Urodela). There are about 350 species of salamanders, included in 54 genera. Salamanders have an ancient fossil lineage, extending back to the Upper Jurassic period, more than 140 million years ago. Like other amphibians, salamanders have a complex life cycle, the stages of which are egg, larva, an…
Starlings are robust, stocky song birds in the family Sturnidae. They have a stout beak and strong legs, and are included with other perching birds in the order Passeriformes. There are about 110 species of starlings, whose natural range includes Eurasia, Africa, the Pacific islands, and Australia. Starlings are small- to medium-sized birds, ranging in body length from about 4-17 in (10-43 cm), an…
Salmon are various species of medium-sized, fusiform (a vertically compressed, torpedo shape) fish with small scales. Their fins are arranged like those of most freshwater fish. On the underside are two pectoral fins, a pair of pelvic fins, an anal fin, and a caudal (or tail) fin. On the back are a dorsal fin and a smaller adipose fin located in front of the tail. The mouth is wide and has numerou…
Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporate. Researchers have noted similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed. …
Marten, sable, and fisher are species of medium-sized carnivores in the family Mustelidae, which also includes the weasels, otters, badgers, minks, skunks, and wolverine. Marten, sable, and fisher are generally solitary animals, living in forests of the Northern Hemisphere. All of these species have highly valuable fur, and are trapped intensively. The American pine marten (Martes americana) range…
Smog refers to an atmospheric condition of atmospheric instability, poor visibility, and large concentrations of gaseous and particulate air pollutants. The word "smog" is an amalgam of the words "smoke" and "fog." There are two types of smog: reducing smog characterized by sulfur dioxide and particulates, and photochemical smog characterized by ozone and …
Salmonella is the common name given to a type of food poisoning caused by the bacteria called Salmonella enteritidis (other types of illnesses are caused by other species of Salmonella bacteria, including typhoid fever). When people eat food contaminated by S. enteritidis, they suffer from inflammation of hte stomach and intestines, with diarrhea and vomiting resulting. This illness is called gast…
Snails are mollusks typically with a coiled, more or less helical, shell as their most conspicuous external feature. When active, snails creep on a broad muscular foot, and display a head with eyes and sensory tentacles. Inside the shell is an asymmetrical visceral mass and one or more gills or lungs used for respiration. Beneath the head is a mouth equipped with a radula, a spiky, long, rasping t…
Salt, the most commonly known of which is sodium chloride, or table salt, is a compound formed by the chemical reaction of an acid with a base. During this reaction, the acid and base are neutralized producing salt, water and heat. Sodium chloride, is distributed throughout nature as deposits on land created by the evaporation of ancient seas and is also dissolved in the oceans. Salt is an importa…
Snakeflies are insects in the family Raphidiidae, in the order Neuroptera, which also contains the closely related alderflies (Sialidae) and dobsonflies (Corydalidae). There are not many species of snakeflies. The approximately 20 species that occur in North America are all western in their distributions. Snakeflies have a complete metamorphosis, with four stages in their life history: egg, larva,…
Matter includes all the material that makes up the universe. It has mass and it takes up space. It includes everything around us: the food we eat, the water we drink, the air we breathe, the ores deep within the earth, as well as the atmosphere above it, the substances that make up the moon, and the stars as well as the dust in the tail of a comet. It is fairly easy to observe that matter exists i…
Saltwater, or salt water, is a geological term that refers to naturally occurring solutions containing large concentrations of dissolved, inorganic ions. In addition, this term is often used as an adjective in biology, usually to refer to marine organisms, as in saltwater fish. The ions with the largest concentrations in marine waters are sodium, chloride, sulfate, magnesium, calcium, potassium, a…
Statistical mechanics is a sub-branch of physics that attempts to explain the behavior of a macroscopic system based on the behavior and properties of that system's microscopic elements. The number of these microscopic elements is usually very large, and it is impossible to accurately predict the behavior of each of these elements as they interact. However, the large number of interactions …
Snakes are limbless reptiles with an elongated, cylindrical body, scaly skin, lidless eyes, and a forked tongue. Most species are non-venomous, some are mildly venomous, and others produce a deadly venom. All snakes are carnivores (or meat-eaters). They are also cold blooded (or ectotherms), meaning their body temperature is determined by the environment, rather than being internally regulated (ho…
The snapdragon or figwort family (Scrophulariaceae), class Dicotyledon, is composed of about 3-4,000 Indian paintbrush (Castilleja sp.). Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission. species and 200 genera of vascular plants. Species in this family occur on all continents except Antarctica, but are most diverse in temperate and mountain ecosystems. Mo…
A sample is a subset of actual observations taken from any larger set of possible observations. The larger set of observations is known as a population. For example, suppose that a researcher would like to know how many hours the average 11th grade student in the United States spends studying English literature every night. One way to answer that question would be to interview a select number (say…
Sand is any material composed of loose, stony grains between 1/16 mm and 2 mm in diameter. Larger particles are categorized as gravel, smaller particles are categorized as silt or clay. Sands are usually created by the breakdown of rocks, and are transported by wind and water, before depositing to form soils, beaches, dunes, and underwater fans or deltas. Deposits of sand are often cemented togeth…
Sand dollars or sea biscuits (phylum Echinodermata, class Echinoidea) are closely related to heart urchins and sea urchins, although they lack the visible long, protective spines of the latter. The body is flattened and almost circular in appearance—an adaptation for burrowing in soft sediment. It is protected by a toughened exterior known as the test, and is covered with short spines. The …
A sandfish is a sand-dwelling lizard of the family Scincidae (a skink) found in desert regions of North Africa and southwestern Asia. It receives the name "sandfish" because it literally "swims" through the loose sand of its preferred habitat. Six or seven species of the genus Scincus are called sandfish. They range from Algeria, in northwestern Africa, to the Sind dese…
Sandpipers are a varied group of shore birds in the family Scolopacidae, order Charadriiformes. The 85 species in this family include the sandpipers, curlews, snipes, woodcocks, godwits, dowitchers, turnstones, and phalaropes. With the exception of Antarctica, this family occurs worldwide. Thirty-seven species in the sandpiper family breed regularly in North America. The smaller species of sandpip…
The sapodilla, Achra zapota, or plum tree is a large evergreen tree native to Central and South America. Sapodilla trees can often grow to 100 ft (30 m) tall with a girth of some 7 ft (2 m). The flowers are white to cream in color and usually open at night. The seeds of these trees are dispersed by bats, which excrete them after consuming the fruit. The durable wood of the sapodilla tree is used i…
Soap is a cleansing agent created by the chemical reaction of a fatty acid with an alkali metal hydroxide. Chemically speaking, it is a salt composed of an alkalimetal, such as sodium or potassium, and a mixture of "fatty" carboxylic acids. The cleansing action of soap comes from its unique ability to surround oil particles, causing them to be dispersed in water and easily rinsed awa…
Statistics is that branch of mathematics devoted to the collection, compilation, display, and interpretation of numerical data. In general, the field can be divided into two major subgroups, descriptive statistics and inferential statistics. The former subject deals primarily with the accumulation and presentation of numerical data, while the latter focuses on predictions that can be made based on…
Sociobiology, also called behavioral ecology, is the study of the evolution of social behavior in all organisms, including human beings. The highly complex behaviors of individual animals become even more intricate when interactions among groups of animals are considered. Animal behavior within groups is known as social behavior. Sociobiology asks about the evolutionary advantages contributed by s…
Sardines are silvery, laterally-flattened fish. They are members of the order Clupeiformes, commonly known as the herring order, and the suborder Clupeoidei. These fish usually live in warm marine waters, are found around the shores of every continent, and are an extremely valuable food fish. There are four families in the order Clupeiformes. Two of the families contain only a single species; one …
Sarin gas (O-Isopropyl methylphosphonofluoridate), also called GB, is one of the most dangerous and toxic chemicals known. It belongs to a class of chemical weapons known as nerve agents, all of which are organophosphates. The G nerve agents (including tabun, sarin and soman), are all extremely toxic, but not very persistent in the environment. Pure sarin is a colorless and odorless gas, and since…
Was there a moment of creation for the universe, or has the universe always existed? The steady-state theory is a cosmological theory for the origin of the universe that suggests the universe has always existed and did not have a moment of creation. This theory was popular during the 1950s and 1960s, but because of observations made during the 1960s, few, if any, astronomers now think that the ste…
While the word "satellite" simply means some object or person that is attendant to another more important object or person, in astronomy it has taken on a much more specific meaning. Here the term refers to any object that is orbiting another larger more massive object under the influence of their mutual gravitational force. Thus any planetary moon is most properly called a satellite…
The chemical element of atomic number 11. Symbol Na, atomic weight 22.9898, specific gravity 0.97, melting point 208°F (97.8°C), boiling point 1,621.4°F (883°C). Sodium is the second element in group 1 of the periodic table. Its chemical symbol reflects its Latin name of natrium. The element was first isolated by the English chemist Sir Humphry Davy in 1807. Only one st…
A steam engine is a machine that converts the heat energy of steam into mechanical energy by means of a piston moving in a cylinder. As an external combustion engine—since it burns its fuel outside of the engine—a steam engine passes its steam into a cylinder where it then pushes a piston back and forth. It is with this piston movement that the engine can do mechanical work. The stea…
Steam pressure sterilization requires a combination of pressure, high temperatures, and moisture, and serves as one of the most widely used methods for sterilization where these functions will not effect a load. The simplest example of a steam pressure sterilizer is a home pressure cooker, though it is not recommended for accurate sterilization. Its main component is a chamber or vessel in which i…
Stearic acid is a chemical compound consisting of an 18 carbon chain whose terminal carbon is connected to an oxygen atom with a double bond and a hydroxyl group (OH) by a single bond. It belongs to class of materials known as fatty acids, produced primarily from natural fats and oils. Stearic acid is an important component in soap and other cosmetic and industrial preparations. Stearic acid is de…
Saturn, sixth planet from the Sun, is the most remote of the planets that were known to premodern astronomers. Saturn is a gas giant with no solid surface; it is 9.45 times wider than Earth and 95 times more massive. It is circled by hundreds of rings consisting of small, ice-covered particles and is also host to at least 30 moons, including Titan, largest moon in the solar system and the only one…
When sodium bicarbonate is heated above 518°F (270°C) it decomposes and produces carbon dioxide. Since carbon dioxide gas is more dense than air, it tends to sink; thus carbon dioxide can smother a fire by obstructing the flow of oxygen to the fuel, which needs oxygen to continue burning. Sodium bicarbonate is employed in fire extinguishers and is widely used on electrical fires. …
A savanna is a plant community characterized by a continuous grassy layer, often with scattered trees or shrubs, that is subject to regular, severe drought and occasional bush fires. A savanna is also the flat, open landscape in which such plant communities thrive. The word savanna comes from the Taino word zabana, which was used to describe a grassy, treeless plain. (Taino was the language of a n…
Steel is the most widely used of all metals, with uses ranging from concrete reinforcement in highways and in high-rise buildings to automobiles, aircraft, and vehicles in space. Steel is iron combined or alloyed with other metals or nonmetals such as carbon. Steel is more ductile (able to deform without breakage) and durable than cast iron and is generally forged, rolled, or drawn into various sh…
It is commonly referred to as soda ash because it was originally obtained from the ashes of burnt seaweed. Now, soda ash is primarily manufactured by a method known as the Solvay process. Currently, it is one of the top industrial chemicals, in terms of volume, produced in the United States. It is mostly used in the manufacture of glass, but is also used in the manufacture of other products and is…
Savants are people with extremely outstanding abilities, often in music, mathematics, memory, or art. Their talents stand in marked contrast to their intelligence in other areas, which is well below normal. For example, a savant who, given any date in the past hundred years, could say what day of the week it fell on, might not be able to perform simple tasks like tying his shoes or catching a bus.…
The mass of a star determines the ultimate fate of a star. Stars that are more massive burn their fuel quicker and lead shorter lives. Because stars shine, they must change. The energy they lose by emitting light must come from the matter of which the star is made. This will lead to a change in its composition. Stars are formed from the material between stars, shine until they exhaust their fuel, …
Sawfish are marine shark-like cartilaginous fish in the family Pristidae in order Rajiformes. Sawfish are characterized by their long snout nose which has sharp teeth on each side. Like other rays, sawfish lurk to attack schools of prey fish with its long snout, and devour the injured fish. The long snout also serves as a defensive weapon, inflicting serious injury on any enemy attacking it. Sawfi…
Stellar magnetic fields are an array of forces that can be observed surrounding and at the surfaces of stars like the Sun. They are similar in nature to the effect of the well-known dipolar magnets found in science laboratories, classrooms, and toys, but far more powerful and infinitely more complex. They are an important part of the physical makeup of stars because they affect their interiors, at…
The saxifrages, currants, and gooseberries are about 40 genera and about 850 species of plants that make up the family Saxifragaceae. These plants occur in all parts of the world, but are most diverse and prominent in arctic, boreal, and montane habitats of North America and Eurasia. The largest genera in the family are the saxifrages (Saxifraga spp.), of which there are about 300 species, most of…
A scalar is a number or measure, usually representing a physical quantity, that is not dependent upon direction. For example, distance is a scalar quantity since it may be expressed completely as a pure number without reference to spacial coordinates. Other examples of scalar quantities include mass, temperature, and time. The term scalar originally referred to any quantity which is measurable on …
Sodium chloride (chemical formula NaCl), known as table salt, rock salt, sea salt and the mineral halite, is an ionic compound consisting of cube-shaped crystals composed of the elements sodium and chlorine. This salt has been of importance since ancient times and has a large and diverse range of uses. It can be prepared chemically and is obtained by mining and evaporating water from seawater and …
Scale insects, mealybugs, or coccids are a diverse group of species of insects in the superfamily Coccoidea, order Homoptera. The females of scale insects are wingless, and are also often legless and virtually immobile. For protection, female scale insects are covered by a scale-like, waxy material. Like other homopterans, scale insects are herbivores with piercing mouth parts that are used to suc…
Magnitude is the unit used in astronomy to describe a star's brightness in a particular portion of the electromagnetic spectrum. Stars emit different amounts of radiation in different regions of the spectrum, so a star's brightness will differ from one part of the spectrum to the next. An important field of research in modern astronomy is the accurate measurement of stellar brightnes…
Sodium hydroxide, NaOH, also known as lye or caustic soda, is an extremely caustic (corrosive and damaging to human tissue) white solid that readily dissolves in water. Sodium hydroxide is used in the manufacture of soaps, rayon, and paper, in petroleum refining, and in homes as drain cleaners and oven cleaners. Sodium hydroxide is one of the strongest bases commonly used in industry. Solutions of…
Sodium hypochlorite (NaOCl) is a chemical compound consisting of sodium, oxygen, and chlorine that has been used for centuries for bleaching and disinfecting. Today, sodium hypochlorite (commonly called chorine bleach) is mass produced by the chlorination of soda ash and is employed in many household products, including laundry bleaches, hard surface cleaners, mold and mildew removers, and drain c…
Stars fall into distinct groups or populations. The basic stellar populations are Population I stars and Population II stars. The sun and most stars near the sun are Population I stars. They are young second- to third-generation stars with compositions that include 2% of elements heavier than hydrogen and helium. Population II stars, on the other hand, are older stars whose compositions are just h…
It is said that Fred Hoyle once described the evolution of a star as a continual war between nuclear physics and gravity. The structure of a star can be characterized as a polarized battle in that war. The gravity of the stellar material pulls on all the other stellar material striving to bring about a collapse. However, the gravity is opposed by the internal pressure of the stellar gas which norm…
Scarlet fever (sometimes called scarletina), is a bacterial disease, so named because of its characteristic bright red rash. Before the twentieth century, and the age of antibiotics, scarlet fever (at one time called "the fever") was a dreaded disease and a leading cause of death in children. The disease is caused by a group A beta-hemolytic streptococcus bacteria (genus Streptococcu…
A scavenger is an animal that seeks out and feeds upon dead and/or decaying organic matter. Some scavengers specialize on feeding upon dead animals, or carrion, while others feed more generally on dead plants and animals. Scavengers are part of the detrital food web of ecosystems. Scavengers provide a very important ecological service, because they help to rapidly reduce dead animals and plants to…
Soil is a complex mixture of pulverized rock and decaying organic matter, which covers most of the terrestrial surface of the Earth. Soil not only supports a huge number of organisms below its surface—bacteria, fungi, worms, insects and small mammals, which all play a role in soil formation—but it is essential to all life on Earth. Soil provides a medium in which plants can grow, sup…
The sun emits a constant stream of particles, mostly protons and electrons, that are known as the solar wind. Many stars also have a similar phenomenon, known as stellar wind. The solar wind is fairly gentle. Stellar winds as gentle as the solar wind are difficult to detect from the earth because other stars are so distant. However, many stars at certain stages in their evolution have very strong …
Soil conservation refers to maintaining the productivity of agricultural land by control of the erosion of soil by wind or water. Soil conservation practices use the land according to its needs and capabilities. Erosion is any process by which soil is transported from one place to another. At naturally occurring rates, land typically loses about one inch (2.5 cm) of topsoil in 100-250 years. A tol…
Schizophrenia is a psychotic disorder (or a group of disorders) marked by severely impaired thinking, emotions, and behaviors. The term schizophrenia comes from two Greek words that mean "split mind." It was coined around 1908, by a Swiss doctor named Eugen Bleuler, to describe the splitting apart of mental functions that he regarded as the central characteristic of schizophrenia. (N…
The solar activity cycle is the periodic, typically 11-year-long variation in the number of active features (for example, sunspots) visible on the Sun's apparent surface or in its atmosphere. Over a period of 11 years, the number of sunspots gradually rises from a low level, reaches a maximum near the midpoint of the cycle, and then declines to a minimum. Solar activity is governed by the s…
Scientific thought aims to make correct predictions about events in nature. Although the predictive nature of scientific thought may not at first always be apparent, a little reflection usually reveals the predictive nature of any scientific activity. Just as the engineer who designs a bridge ensures that it will withstand the forces of nature, so the scientist considers the ability of any new sci…
A solar flare is a sudden, localized release of energy in the sun's outer atmosphere. This energy, in the form of radiation, is distributed throughout the electromagnetic spectrum, allowing flares to be seen at many different wavelengths, from the x ray to the radio regions. A solar flare erupting from the chromosphere of the sun. NASA/Science Photo Library/Photo Researchers, Inc. Repro…
The scorpion fly, despite its name, is neither a scorpion nor a fly. The name is a suggestion of the general appearance of the insect. They have four membranous wings that are the same size and shape. The head is rather elongated and points down in a beak-like fashion with the chewing mouthparts located at the tip of the beak. The genital segment of the male scorpion fly has an enlarged, rounded a…
Earth rotates about its polar axis as it revolves around the Sun. Earth's polar axis is tilted 23.5° to the orbital plane (ecliptic plane). Combinations of rotation, revolution, and tilt of the polar axis result in differential illumination and changing illumination patterns on Earth. These changing patterns of illumination result in differential heating of Earth's surface tha…
Scorpionfish are ray-finned bony marine fish belonging to the family Scorpaenidae. Most of the 300 species of scorpionfish live in the seas around North America. A major anatomical characteristic of scorpionfish is a bony structure extending from the eye to the operculum or gill cover. The common name of scorpionfish refers to the spiny condition of the members of this family which includes extrem…
Solar prominences are large, glowing clouds of gas suspended in magnetic field loops above the Sun's photosphere. Although impossible to see in white light (the brilliance of the photosphere blots them out), they are easily visible in hydrogen alpha images (pictures taken in light emitted by hydrogen atoms, the principal constituent of the Sun). Prominences have been observed during eclipse…
Screamers are three species of large birds in the family Anhimidae. This family is in the order Anseriformes, which also includes the ducks, geese, and swans, although screamers bear little superficial resemblance to these waterfowl. Screamers are non-migratory birds that inhabit a wide range of aquatic habitats in the tropics of South America, especially marshy places. Screamers are large birds, …
Screwpines are shrubs, trees, or vines belonging to the family Pandanaceae in order Pandanales, and the class Arecidae, which also includes the palms. Screwpines are native to the tropics of South and Southeast Asia, northern Australia, and west Africa. Despite their common name, screwpine are not related to the true pines, which are gymnosperms of the phylum Coniferophyta. Screwpines are common e…
The sculpins are about 300 species of small, rather grotesquely shaped fish that make up the family Cottidae. Most species of sculpins occur in cold or cool-temperate marine waters of the Northern Hemisphere, but a few species occur in fresh waters of northern Asia, Europe, and North America. Sculpins are short, stout-bodied fishes, with a large and broad head, large eyes, a large mouth, and broad…
Sea anemones are invertebrate animals belonging to the phylum Coelenterata, a term that means hollow gut. Sea anemones are found in all major oceans from the polar regions to the equator. All are exclusively marine-dwelling with a strong tendency for shallow, warm waters. More than 1,000 species have been described so far. These vary considerably in size, with a body diameter that ranges from just…
Sea cucumbers are echinoderms, belonging to the class Holothuroidea of the order Echinodermata. About 1,000 species have been described, which vary in size from only 1.2 in (3 cm) to more than 3.3 ft (1 m) in length. Sea cucumbers occur in all of the oceans, being found in waters up to 655 ft (200 m) in depth, and perhaps deeper. In appearance, these animals range from an almost spherical to long …
The solar system comprises the Sun, nine major planets, some 100,0000 asteroids larger than 0.6 mi (1 km) in diameter, and perhaps 1 trillion cometary nuclei. While the major planets lie within 40 astronomical units (AU) of the Sun, the outermost boundary of the solar system stretches to 1 million AU, one third the way to the nearest star. It is believed that the solar system was formed through th…
The comparative study of religion has generally focused on doctrine, on canonical texts and their interpretations. But what of religions without writing, much less canonical texts? …
Sea horses are bony fish (or teleosts) in the family Syngnathidae, which includes about 230 species in 55 genera, most of which are pipefishes. The "true" sea horses comprise some 25 species in the genera Hippocampus and Phyllopteryx, which make up the subfamily Hippocampinae. Species of sea horses occur in warm-temperate and tropical waters of all of the world's oceans. The u…
Romanticism is perhaps the richest and certainly the most vexed of the "isms." At the most general level, the term denotes a set of common tendencies in European art and thought from about 1797 to 1848. Ultimately those tendencies influenced the arts, especially literature, in virtually every country from Spain to Russia, but their acknowledged origins and centers were Britain, Franc…
To most people sea level is the point at which the surface of the land and sea meet. Officially known as the sea level datum plane, it is a reference point used in measuring land elevation and water depths. It refers to the vertical distance from the surface of the ocean to some fixed point on land, or a reference point defined by people. Sea level became a standardized measure in 1929. Mean sea l…
In order to define and explain the paired concepts of sacred and profane, it is important to look at these concepts as developed in the influential work of the French sociologist Émile Durkheim (1858–1917). …
Resembling a plant more than an animal, sea lilies are some of the most attractive but least-known animals of the deep oceans. Sea lilies are members of the class Crinoidea (phylum Echinodermata), a class that also includes the feather stars. Sea lilies are also related to more familiar echinoderms such as sea urchins, starfish, and sea cucumbers. Unlike these small, squat forms, however, the main…
Every civilization has its share of sacred places, that is, geographical locations, buildings, monuments, or environmental features, such as mountains, lakes, rocks, waterfalls, and so on, that are believed to be endowed with intense spiritual qualities. Indeed, such places are frequently thought to possess a variety of supernatural powers that can heal, rejuvenate, or otherwise affect the human b…
Sea lions are large marine mammals in the family Otariidae, sub-order Pinnipedia, order Carnivora, found now along the Pacific and South Atlantic coasts and on many islands of the southern hemisphere. Sea lions may have appeared first on the Pacific shores during the Lower Miocene. They are less fully adapted to aquatic life than are the true seals (family Phocidae of the same sub-order Pinnipedia…
The solar wind is a continuous stream of particles that flows outward from the Sun through the solar system. The particles escape from the Sun because its outer atmosphere is very hot, and the atoms there move too rapidly for the Sun's gravity to hold onto them. The solar wind, which is made mainly of ionized hydrogen (free protons and electrons), flows away from the Sun at a velocity of se…
A more restrictive approach to questions of textual authority was presented in the Great Instruction Discourse (Mahapadesa-sutta, attributed to the Buddha but probably written after his death), in which he advises his followers to compare contested teachings with the corpus of discourses known to have been spoken by the Buddha. If a teaching or text accords with the oral instructions (Pali, sutta;…
Sea moths are small fish of the family Pegasidae, order Pegasiformes, subclass Actinopterygii, class Osteichthyes. They are characterized by very large wing-like pectoral fins, which make them look like moths. They are found only in tropical Indian and West Pacific Oceans where they live mainly on sandy bottoms. There are about six species, of which Pegasus volitans is typical and reaches about 6 …
The traditional biography of Muhammad portrays the first encounter of this forty-year-old merchant with God's messenger Gabriel in a mountain cave as a terrifying experience. He is commanded to recite, yet does not know how or what to recite. Indeed, for Muslims the illiteracy, or at least lack of education, on the part of the Prophet is a guarantee that the revelations were not composed by…
Sea spiders (phylum Arthropoda, class Pycnogonida) are a group of arthropods that take their common name from their superficial resemblance to the true spiders. Although rarely seen, these are widespread animals occurring in every ocean, with a preference for cooler waters. Sea spiders occupy a wide range of habitats: some species have been recorded from a depth of 19,685 ft (6,000 m), but the maj…
Sage philosophy is a body of knowledge attributed to wise men and women in communities and is regarded to be philosophically significant for both its content and its critical approach to the sustenance and growth of knowledge at the communal level. Although the term came into use rather recently in the course of African philosophers' appraisal of the nature and relation of philosophy to the…
Classified within the same phylum (Chordata), sea squirts and salps belong to separate classes, the Ascidiacea and Thaliacea, respectively. Both groups are also known as tunicates, a group of primitive chordates which have a primitive feature known as the notochordthe earliest and simplest equivalent to the vertebrae of more developed animals. In appearance adult sea squirts and salps are barrel-s…
Abundance and scarcity are crucial to Latin America because of the sharp disparities between the wealth produced in this region and the impoverishment of people and environment. Though ideas about causes differ sharply, Latin American social thought has revolved in substantial part around this axis. This entry begins with ideas from before the Spanish conquest, proceeds to key debates among the co…
Sea urchins (phylum Echinodermata) are small marine species that have a worldwide distribution. All are free-living and solitary in nature; some 800 species have been identified to date. The body is characterized by its rounded or oval shape and, in most species, by the presence of large numbers of sharp spines of varying lengths. The underside is usually flattened in contrast to the convex upper …
Scholasticism is best understood not as a set of doctrines, but as a method, or body of intellectual practices. In particular, Scholasticism developed as the method through which Christian thinkers of the patristic and medieval periods gradually transformed the narratives of Scripture into a theological system. Scholastic method, then, has its roots in the earliest Christian times. It reached its …
Soldering is the process by which two pieces of metal are joined to each other by means of an alloy. The tool used to make this kind of joint is called a soldering iron, and the alloy from which the connection is made is called a solder. Soldering can be used for making either a mechanical or an electrical connection. An example of the former case is the situation in which a plumber uses plumbers …
Science is a product, a particular kind of knowledge, as well as a process to obtain such knowledge. It is a social activity primarily aimed at obtaining objective and consensual knowledge about nature and to use this knowledge for socially desirable purposes. Scientists and philosophers disagree about whether scientific knowledge mirrors nature as it really is, or if it merely constitutes an inst…
The term solstice refers to the two dates of the year on which the Sun reaches its northernmost and southernmost declinations (declination is the celestial equivalent of latitude). During the spring we frequently hear someone remark that "the days are getting longer," or during the fall that they are getting shorter. This phenomenon occurs because Earth's rotational axis is ti…
The most familiar characteristics of modern science—rigorously demonstrated relationships based on a combination of experimentation and exact measurement—did not exist anywhere in the world before the seventeenth century. Any definition of science that holds for earlier times or for places other than Europe must be more inclusive. More useful criteria are the attempt to find rational…
The history of science as an academic discipline grew in the post-1945 era along with the development of higher education and the expansion of science and technology. In the United States, Vannevar Bush's seminal 1945 article, "Science, the Endless Frontier: A Report to the President," called for a government-supported national research foundation. Bush's influence led …
Seals are large carnivorous marine mammals in the order Pinnipedia that feed on fish, squid, and shell-fish; some even feed on penguins. They are aquatic animals that spend time on shores and ice floes. Seals have streamlined bodies and webbed digits, with the forelimbs acting as flippers, while the hind limbs are backwardly directed in swimming and act as a propulsive tail. A small tail is also p…
Like a rapacious and relentlessly predatory science fictional entity assimilating all it encounters (the Star Trek Borg come to mind), science fiction in the early twenty-first century is an unstoppable expansive force that is certainly not limited to one particular genre. Science fiction literature, once ghettoized and marginalized, is pervasive and ever more rapidly garnering respect. PMLA, Amer…
Seamounts are submarine mountains, often volcanic cones, that project 150-3,000 ft (50-1,000 m) or more above the ocean floor. They are formed primarily by rapid undersea buildups of basalt, a dark, fine-grained rock that is the main component of the ocean's crust. Seamounts form by submarine volcanism. After repeated eruptions, the volcano builds upwards into shallower water. If a seamount…
As distinct from the centuries-old conception of scientific revolutions (plural), the concept of the one and only Scientific Revolution dates from the 1930s. What meaning, if any, to give the concept over and above its signifying the birth of modern science has been in lively, productive dispute ever since. This entry first outlines the historiographical vagaries of the term, then (from the headin…
Seasonal winds are movements of air repetitively and predictably driven by changes in large-scale weather patterns. Seasonal winds occur in many locations throughout the world. The name assigned to a particular seasonal wind—and the underlying physical forces that drive the winds—depend upon the unique geographic location. One of the most commonly recognized seasonal winds are the mo…
Solubility in the general sense refers to the property of being soluble—being able to dissolve, usually in a liquid. Chemists, however, use the word solubility to also mean the maximum amount of a chemical substance that dissolves in a given amount of solvent at a specific temperature. How much sugar could you dissolve in a cup of hot coffee? Certainly one teaspoonful would mix into the liq…
The terms secularization and secularism have had a variety of meanings since they were coined; "secularization" in the mid-seventeenth century and "secularism" in the mid-nineteenth, both incorporating the word secular. "Secular," from the Latin "saeculum," a generation or age, originally referred to secular clergy who were not in a monastic …
Seasons on Earth are found only in the temperate zones. These zones extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. In these regions of Earth nature exhibits four seasons; spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature, amounts of precipitation, and the length of daylight. Spring comes from an …
A solution is a homogenous (uniform throughout) mixture, on a molecular level, of two or more substances. It is formed when one or more substances are dissolved in one or more other substances. The scientific nature of solutions is a relatively recent discovery, though solutions in one form or another have been used by people throughout history. The substances (solids, liquids, or gasses) in a sol…
The concept of segregation is usually based on race, gender, class, religion, or ethnicity, depending on the circumstances under which it is practiced. This entry will mainly address racial and ethnic segregation and when appropriate address how it spawned gender and class segregation in the Americas and Africa. …
Secondary pollutants are not emitted directly to the air, water, or soil. Secondary pollutants are synthesized in the environment by chemical reactions involving primary, or emitted chemicals. The best known of the secondary pollutants are certain gases that are synthesized by photochemical reactions in the lower atmosphere. The primary emitted chemicals in these reactions are hydrocarbons and gas…
Unwanted sexual attention was imposed on people in no position to refuse it long before sexual harassment was a recognized idea. Sexual harassment seems to be practiced wherever the sexes are materially unequal, which generally they are; its reality in paid work can be traced from the beginnings of industrialization and in unpaid work throughout slavery. Until the mid-1970s, it had no name. The wo…
The secretary bird (Sagittarius serpentarius) is the only member of the family Sagittariidae. This family is part of the Accipitriformes, which includes other hawk-like raptors such as hawks, eagles, vultures, kites, falcons, and the osprey. The secretary bird is native to sub-Saharan Africa, and occurs in open grasslands and savannas. The species is wide-ranging, and some populations are nomadic,…
When we think of sexuality, we think of many different things. We think of reproduction and the different bodies and reproductive capacities of men and women. We think of pleasure, the pleasures of the body, but also the pains, mental and physical, that can wrack the body. We think of love, and the joys of human involvement, but we might also remember the fear and hate that sexuality can evoke…
Islam considers sexual pleasure to be a gift from God to humanity that should be enjoyed with gratitude. The religion has also always frowned on celibacy. The Prophet, who is an exemplar for all aspects of Islamic behavior, enjoyed his many wives, including his famous child-bride, 'A'isha. Hence, sexual regulation is concerned not so much with the potential for sexuality to distract …
The phrase sexual orientation is used to describe different forms of erotic attraction: toward people of the same gender (homosexual), the opposite gender (heterosexual), or both (bi-sexual). Like any simplistic categorization, such definitions quickly become mired in contradictions and complications. For instance, is the label "heterosexual" to be reserved for people who only have s…
The solution of an equation is the set of all values which, when substituted for unknowns, make an equation true. For equations having one unknown, raised to a single power, two fundamental rules of algebra, including the additive property and the multiplicative property, are used to determine its solutions. Solutions for equations with multiple unknown variables are found by using the principles …
Shinto, composed of two ideographs, literally means the "way of the kami." Although kami can be translated as gods or deities, it also refers more generally to spirit-beings, the supernatural, or to a sacred quality in which an individual can even participate. Shinto refers to what has become a religious tradition indigenous to Japan that recognizes the existence of the kami governi…
Sedges are monocotyledonous plants in the genus Carex that make up most of the species in the family Cyperaceae. This family consists of about 4,000 species distributed among about 90 genera, occurring world-wide in moist habitats in all of the major climatic zones. The sedges are the largest group in the family with about 1,100 species, followed by the papyrus or nut-sedges (Cyperus spp.; 600 spe…
Skepticism is both a generalized sense of doubt and disbelief as expressed in everyday language and an identifiable school of thought in the history of ideas. In its most general sense it refers to uncertainty, doubt, disbelief, suspension of judgment, and rejection of knowledge. It is characterized by its opposition to dogmatism, which claims to know reality and the truth. …
Slavery is possibly the most ubiquitous of all human institutions. It has existed in most times and most places, and few peoples have not, at various times, been either the enslaved or the enslavers. While slavery has generally been coerced, the result of war capture or kidnapping, in periods of Slaves cutting sugar cane in Antigua, 1823 painting by William Clark. The majority of slave labor w…
Sociability is a descriptive term that refers to the relational or interactive disposition and condition from which persons, as individuals, are said to derive their identity and status. According to this view, humans are regarded as beings whose defining characteristics, like reason, and its manifestation in cognitive, moral, and language capacities, are said to arise from the social conditioning…
SONAR, an acronym for Sound Navigation And Ranging, is a technique based on echolocation used for the detection of objects underwater. …
The term social capital first began to be defined in the 1970s and remained largely restricted to the academic world of the social sciences until the 1990s, when it suddenly emerged as a central element in public discussions and policy debates about the quality of civic culture in Western nations, especially the United States of America. At the same time it also gained a place of prominence in dis…
Song birds are any birds that sing musically, almost all of which are in the suborder Oscines of the order Passeriformes, or perching birds. Passeriform birds have feet adapted for gripping branches, plant stems, and similar perches, and they comprise about one-third of living bird families, and one-half of the species. A major function of singing in birds is to proclaim the location and limits of…
Modern theorists of the social contract school argued that political authority was artificial and conventional rather than divinely or naturally ordained. To sustain their argument, some—including Thomas Hobbes (1588–1679), John Locke (1632–1704), and Jean-Jacques Rousseau (1712–1778)—appealed to a social contract as a way of explaining the rational basis for and…
Sonoluminescence is the emission of light from bubbles of air trapped in water which contains intense sound waves. Hypothesized in 1933 by Reinhardt Mecke of the University of Heidelberg, from the observation that intense sound from military sonar systems could catalyze chemical reactions in water, it was first observed in 1934 by H. Frenzel and H. Schultes at the University of Cologne. Modern exp…
Social Darwinism arose in the last quarter of the nineteenth century. It was an intellectual movement associated with the theory of evolution in general but was principally derived from the works of Charles Darwin (1809–1882), especially his Origin of Species (1859). Five major questions are raised by the extension of Darwin's theories to the human sphere. 1. To what extent was Darwi…
Sorghum (genus Sorghum) refers to various species of grasses (family Poaceae) that are cultivated as food crops. Because the relationships among the various species and their hybrids are highly complex and not well understood, the cultivated grain sorghums are usually named as Sorghum bicolor. Sorghum is a tropical grass, well adapted to high productivity in a hot and dry climatic regime, and wate…
The "new" social history that emerged in the United States in the 1960s and came to dominate the profession by the 1980s, conjoined two scholarly agendas. The first was a program for reshaping history along the lines of the behavioral sciences—in particular economics, political science, and sociology—and employing quantitative methods and models in historical research. …
The difficulty of defining socialism is apparent to anyone who attempts to study this protean doctrine, not least because what socialism is or is not is usually a matter of contentious debate. However, there is a general consensus that the various schools of socialism share some common features that can be summarized as follows. Socialism is above all concerned with the relationship between the in…
Socialist ideas have been in Africa before the advent of colonialism at the turn of the nineteenth century. African socialisms represent various combinations of African thinkers, politicians, and activists' absorption with and reconfiguring of nineteenth-and twentieth-century European socialist ideas and practice. The sources are multiple, from trades unions and contact with European worker…
Sound waves are pressure waves that travel through Earth's crust, water bodies, and atmosphere Sound waves induce vibration in a body (e.g., the tympanic membrane of the ear) or are produced as a result of vibration of that body. Sound waves can be detected and interpreted by instrumentation (e.g., by a seismograph), or by variety of pressure sensitive organs in living beings (e.g., the lat…
The concept of society is in transition due to globalization and the knowledge associated with it. Today's views of society are not altogether new, however. Rather, they are engaged with the rediscovery of the worlds of antiquity. Since the 1970s, scholars have reached to the past to explain mainstay concepts such as society. In this manner, ancient Eastern ideas flood into the current West…
Sophistic speeches were famous in antiquity for their rhetorical style and their moral and philosophical content. Prodicus' "Hercules at the Crossroads," (see Xenophon, Memorabilia, II.i.21–34) depicts a young Hercules at the brink of manhood choosing between a life of virtue and one of vice. Though the life of vice appears easy at first and the life of virtue difficult…
Sovereignty refers to the supreme and ultimate source of authority that exists within any political unit or association. A sovereign power is deemed independent of all other authorities and it possesses no rivals within its jurisdiction. Thus, sovereignty has internal and external dimensions. Internally, it connotes the superior and final power to determine who shall rule and how rule shall occur.…
"What do they know of cricket who only cricket know?" queried the historian and political reformer C. L. R. James in the preface of Beyond a Boundary (1963), his lyrical exploration of political, social, and racial relations in the twentieth-century West Indies and British Empire. He answered throughout the book that those who only know sport really know nothing of value. Sport, he i…
The concept of the state was central to the social sciences until temporarily displaced in the 1950s by a concept of the "political system" that is mainly associated with Talcott Parsons's (1902–1979) systems analysis. Parsons's sociology identified the political system with behaviors and institutions that provide a center of integration for all aspects of the so…
Sediments are loose Earth materials such as sand that accumulate on the land surface, in river and lake beds, and on the ocean floor. Sediments form by weathering of rock. They then erode from the site of weathering and are transported by wind, water, ice, and mass wasting, all operating under the influence of gravity. Eventually sediment settles out and accumulates after transport; this process i…
The scope of coverage of the term postcolonial varies across disciplinary fields and authors, being broader in literary studies, for example, than in political science. Some authors include former settler colonies as referents alongside non-settler colonies. Other analysts, such as Amina Mama, distinguish the term postcolonial, used to refer specifically to former colonies, from the term post-impe…
The state of nature is a situation without government, employed in social contract theory in order to justify political authority. The device is most important in the works of the great contract theorists of the seventeenth and eighteenth centuries, mainly Thomas Hobbes (1588–1679), John Locke (1632–1704), and Jean-Jacques Rousseau (1712–1778). But it has a long history and wa…
German–American physicist Albert Einstein's (1879–1955) theory of relativity consists of two major portions: the special theory of relativity and the general theory of relativity. Special relativity deals with phenomena that become noticeable when traveling near the speed of light and reference frames that are moving at a constant velocity, inertial reference frames. General r…
A sedimentary, or depositional, environment is an area on the Earth's surface, such as a lake or stream, where large volumes of sediment accumulate. All environments of deposition belong to one of three settings: terrestrial, coastal (or marginal marine), and marine. Subenvironments, each with their own characteristic environmental factors and sedimentary deposits, make up a sedimentary env…
Sedimentary rocks form at or near the Earth's surface from the weathered remains of pre-existing rocks or organic debris. The term sedimentary rock applies both to consolidated, or lithified sediments (bound together, or cemented) and unconsolidated sediments (loose, like sand). Although there is some overlap, most sedimentary rocks belong to one of the following groups-clastic, chemical, o…
The seed ferns are an extinct group of plants known technically as the Pteridospermales. As indicated by their name, the seed ferns had leaves which were fernlike in appearance, and they reproduced by making seeds. Some seed ferns resembled tree ferns (family Cyatheaceae), a still-living group of tropical plants which are treelike in appearance but which reproduce by making spores. The seed ferns,…
The South American continent stretches from about 10° above the equator to almost 60° below it, encompassing an area of about 7 million sq mi (18 million sq km). It is divided into ten countries. The continent can be divided into three main regions with distinct environmental and geological qualities: the highlands and plateaus of the east, which are the oldest geological feature in …
The soybean (Glycine max) is a domesticated species in the pea family (Fabaceae). Like other cultivated species in this family, soybean has symbiotic Rhizobium bacteria growing in nodules on its roots. These bacteria fix atmospheric nitrogen gas into ammonia and allow the crop to grow with relatively little additional fertilization of this key nutrient. Soybean is an annual, dicotyledonous plant. …
Space is the three-dimensional extension in which all things exist and move. We intuitively feel that we live in an unchanging space. In this space, the height of a tree or the length of a table is exactly the same for everybody. Einstein's special theory of relativity tells us that this intuitive feeling is really an illusion. Neither space nor time is the same for two people moving relati…
Seeds are the products of the sexual reproduction of plants, and for this reason the genetic information of seeds is influenced by both of the parents. Sexual reproduction is important for two reasons. The first involves the prevention of the loss of potentially important genetic information, a process that occurs when non-sexual means of propagation are prevalent. The other benefit of sexual repr…
Segmented worms (phylum Annelida) are so named because of their elongated, more or less cylindrical bodies divided by grooves into a series of ringlike segments. Typically, the external grooves correspond to internal partitions called septa, which divide the internal body space into a series of compartments. Perhaps the most familiar examples of segmented worms are the common earthworms or night c…
A space probe is any uncrewed, instrument-carrying spacecraft designed to travel to an extraterrestrial environment beyond Earth orbit. The first recorded mention of a possibility of a true space probe dates to 1919, when United States physicist and rocketry pioneer Robert Goddard (1882–1945) suggested that a flash from an explosion produced by a rocket on the Moon's surface could be…
Seismographs today consist of three essential parts. One is a seismometer, a device (like the seismoscope) that detects earth movements. A second component is a device for keeping time so that each earth movement can be correlated with a specific hour, minute, and second. The third component is some device for recording the earth movement and the time at which it occurred. The written record produ…
Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to raise their own reproductively successful offspring. As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is t…
A sequence is an ordered list of numbers. It can be thought of as a function, f(n), where the argument, n, takes on the natural-number values 1, 2, 3, 4,... (or occasionally 0, 1, 2, 3, 4,...). A sequence can follow a regular pattern or an arbitrary one. It may be possible to compute the value of f(n) with a formula, or it may not. Another sequence is the sequence of prime numbers: 2, 3, 5, 7, 11,…
Sequencing refers to the biotechnology techniques that determine the order of the genetic material. The genetic material that acts as the blueprint for most cells and organisms is deoxyribonucleic acid (DNA). DNA provides the information to make ribonucleic acid (RNA), which in turn provides the information to produce protein. The information for all living things is stored in the genetic material…
Sequoias are species of coniferous trees in the genus Sequoia, family Taxodiaceae. Sequoias can reach enormous height and girth and can attain an age exceeding 1,000 years. These giant, venerable trees are commonly regarded as botanical wonders. About 40 species of sequoias are known from the fossil record, which extends to the Cretaceous, about 60 million years ago. At that time, extensive forest…
The space shuttle is a reusable spacecraft that takes off like a rocket, orbits the Earth like a satellite, and then lands like a glider. The space shuttle has been essential to the repair and maintenance of the Hubble Space Telescope and to construction of the International Space Station; it has also been used for a wide variety of other military, scientific, and commercial missions. It is not ca…
The name servomechanism means, quite literally, slave machine. A servomechanism is a physical device that responds to an input control-signal by forcing an output actuator to perform a desired function. Servomechanisms are often the connection between computers, electronics, and mechanical actions. If computers are the brains, servomechanisms are the muscles and the hands that do physical work. Se…
Sesame are plants in the genus Sesamum, family Pedaliaceae, which are grown for their edible seeds and oil. Sesame is native to Africa and Asia, and was brought to North America from Africa during the slave trade. There are about 15 species of sesame, but only two, S. indicum and S. orientale, are cultivated for commercial purposes. Evidence has shown that sesame has been used for thousands of yea…
Set theory is concerned with understanding those properties of sets that are independent of the particular elements that make up the sets. Thus the axioms and theorems of set theory apply to all sets in general, whether they are composed of numbers or physical objects. The foundations of set theory were largely developed by the German mathematician George Cantor in the latter part of the nineteent…
Since radio astronomers first tuned into the skies, scientists have listened for an elusive radio signal that would confirm the existence of extraterrestrial life. One of the major efforts in the last quarter of the twentieth century was a project termed the Search for Extraterrestrial Intelligence (SETI). Over the years the SETI project evolving into a variety of programs utilizing research resou…
Severe acute respiratory syndrome (SARS) is the first emergent and highly transmissible viral disease to appear during the twenty-first century. Patients with SARS develop flu-like fever, headache, malaise, dry cough and other breathing difficulties. Many patients develop pneumonia, and in 5-10% of cases, the pneumonia and other complications are severe enough to cause death. SARS is caused by a v…
Sewerage and sewage must be defined at the outset because they are often used incorrectly. Sewerage is a system of pipes used to collect and carry sewage, which is the wastewater discharged from domestic premises. Domestic sewage consists of human wastes, paper, and vegetable matter. This type of waste is organic because it consists of compounds containing carbon and can be broken down by microorg…
A sewing machine is a mechanical device equipped with a needle (or needles) threaded at the point-end, which puncture the fabric periodically as it moves under the needle; each stitch is created as the thread loops onto itself (chain stitch) or locks around a second strand of thread (lock stitch), sewing the fabrics together. Sewing machines are used in both the home and industry, but are designed…
The typical sparrows, buntings, and their allies are 281 species of birds that comprise the subfamily Emberizinae, family Emberizidae. The emberizid sparrows and buntings occur in a great variety of habitats, and are widely distributed, occurring on all of the habitable continents except for Southeast Asia and Australia. The greatest diversity of species, however, occurs in the Americas. The phylo…
Sex change, also called transsexuality, is a procedure by which an individual of one sex is hormonally and surgically altered to attain the characteristics of the other sex. A male is changed into a female or a female into a male, complete with altered genitalia and other secondary sex characteristics. It has been estimated that one male in every 20,000-30,000 wants to become female. The number of…
The optical instruments called sextants have been used as navigation aids for centuries, especially by seafarers. In its simplest form, a sextant consists of an eyepiece and an angular scale called the "arc," fitted with an arm to mark degrees. By manipulating the parts, a user can measure the angular distance between two celestial bodies, usually Earth and either the Sun or Moon. Th…
The most widely accepted definition of a species is the biological species concept proposed by Ernst Mayr in the 1940s. A species is a population of individual organisms that can interbreed in nature, mating and producing fertile offspring in a natural setting. Species are organisms that share the same gene pool, and therefore genetic and morphological similarities. …
Although the composition of most stars is very similar, there are systematic variations in stellar spectra based on their temperatures. A typical star has a spectrum consisting of a continuous range of colors overlaid with dark lines. The positions, strengths, and shapes of these lines are determined by the temperature, density, gravitational fields, velocity, and other properties of the star. In …
Sexual reproduction is the process through which two parents produce offspring which are genetically different from themselves and have new combinations of their characteristics. This contrasts with asexual reproduction, where one parent produces offspring genetically identical to itself. During sexual reproduction, each parent contributes one haploid gamete (a sex cell with half the normal number…
A spectral line is light of a single frequency, or wavelength, which is emitted by an atom when an electron changes its energy level. Because the energy levels of the electron vary from element to element, scientists can determine the chemical composition of an object from a distance by examining its spectrum. In addition, the shift of a spectral line from its predicted position can show the speed…
A spectroscope is an instrument used to observe the atomic spectrum of a given material. Because atoms can absorb or emit radiation only at certain specific wavelengths defined by electron transitions, the spectrum of each type of atom is directly related to its structure. There are two classifications of atomic spectra: absorption and emission. An absorption spectrum is produced when light passes…
The absorption, emission, or scattering of electromagnetic radiation by atoms or molecules is referred to as spectroscopy. A transition from a lower energy level to a higher level with transfer of electromagnetic energy to the atom or molecule is called absorption; a transition from a higher energy level to a lower level is called emission (if energy is transferred to the electromagnetic field); a…
A simple machine is a device for doing work that has only one part. Most authorities list six kinds of simple machines: levers, pulleys, wheels and axles, inclined planes, wedges, and screws. One can argue, however, that these six machines are not entirely different from each other. Pulleys and wheels and axles, for example, are really special kinds of levers, and wedges and screws are special kin…
Sexually transmitted diseases (STDs) are diseases that are contracted through sexual contact. STDs are caused by a wide range of organisms including viruses, bacteria, chlamydiae, mycoplasma, fungi, protozoa, and arthropods. STDs remain epidemic in all societies and the range of known STD causng pathogens continues to increase. Long known as venereal disease, after Venus, the Roman goddess of love…
Certain properties of objects or physical processes, such as the frequency of light or sound, the masses of the component parts of a molecule, or even the ideals of a political party, may have a wide variety of values. The distribution of these values, arranged in increasing or decreasing order, is the spectrum of that property. For example, sunlight is made up of many different colors of light, t…
Speech is defined as the ability to convey thoughts, ideas, or other information by means of articulating sound into meaningful words. Many animals can make sounds and some can tailor these sounds to a given occasion. They may sound an alarm that a predator is in the area, warning others of their species that something has trespassed into their territory. Animals may make soothing sounds to let of…
A sphere is a three dimensional figure that is the set of all points equidistant from a fixed point, called the center. The diameter of a sphere is a line segment which passes through the center and whose endpoints lie on the sphere. The radius of a sphere is a line segment whose one endpoint lies on the sphere and whose other endpoint is the center. A great circle of a sphere is the intersection …
The sharks are a group of about 350 related species of cartilaginous fish, members of which are found in every ocean in the world. Far from their reputation as primitive monsters, the sharks are, in fact, some of the most fascinating, well-adapted marine organisms. Their many structural and functional adaptations, such as their advanced reproductive systems and complex sensory abilities, combine t…
Sheep are ruminant members of the Bovidae family. They belong to the genus Ovis, which contains three species, Ovis musimon, Ovis orientalis, and Ovis aries. Spanish farmers developed the Merino breed of sheep in the sixteenth and seventeenth centuries, and the fineness of its wool is unsurpassed even today. In the seventeenth century Robert Bakewell, in England, using his newly discovered breedin…
In archaeology, the term shell midden analysis refers to the study of marine shell valves that were once used as food by prehistoric peoples. In the United States, North American Indian tribes who lived near coastal areas often collected clams, oysters, mussel, and other species of shellfish to supplement their diets. Once the meat was extracted, the remaining shells were sometimes used to make or…
Shingles, also known as herpes zoster, are small, painful skin lesions caused by the same virus that causes chicken pox, the varicella zoster virus (VZV). Shingles usually occur in older individuals and in people who have weakened immune systems, such as organ transplant patients taking drugs to suppress their immune systems or people with acquired immune deficiency syndrome (AIDS). Shingles occur…
Shore birds, sometimes called waders, include representatives from a number of families in the order Charadriiformes, including plovers (Charadriidae), oystercatchers (Haematopodidae), avocets and stilts (Recurvirostridae), jacanas (Jacanidae), and sandpipers, snipe, phalaropes, and their close relatives (Scolopacidae). Despite their classification in the same order, shore birds are not closely re…
Spider monkeys are slender, medium-sized monkeys with long limbs and very long tails. They live in trees, rarely coming down to the jungle floor. They are very adept at moving around in trees with the help of their prehensile tails; "prehensile" is a term that means their tails are well adapted for holding on to objects. These monkeys inhabit a territory ranging from southern Mexico …
Ribonuclease (RNase) is the name of a group of enzymes that change ribonucleic acid (RNA) by digesting (cutting) phosphorus-oxygen bonds. The RNases are the subject of wide investigation in the laboratory, though scientists are still learning the many ways they work in living cells. The best-studied RNase is from the pancreas of cattle. Its main portion, called ribonuclease A, was the first enzym…
The spiderwort family (Commelinaceae) is a small family of monocotyledonous (with one seed leaf) plants, found primarily in tropical and desert areas of the world. The family contains 38 genera and about 600 species. All members of the family are herbaceous, and are easily recognized by their simple, linear leaves, and large, brittle nodes. Their flowers are borne either on a terminal inflorescenc…
Shoreline protection is the engineering effort designed to lessen or eliminate coastal erosion. Because sea level is rising and we have chosen to develop coastal areas, shoreline erosion has become a common and urgent problem for many communities. In essence, shoreline protection consists of engineered structures or other solutions meant to slow erosion by rising sea levels and storm wave action. …
Shotgun cloning (also known as the shotgun method) is a method to duplicate genomic DNA. The DNA to be cloned is cut using a restriction enzyme or by randomly using a physical method to smash the DNA into small pieces. These fragments are then taken together and cloned into a vector. The original DNA can be either genomic DNA (whole genome shotgun cloning) or a clone such as a YAC (yeast artificia…
Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is composed of nucleic acids that are found in the nucleus of plants and animals. Nucleic acids consist of high–molecular–weight macromolecules, which are made up of hundreds or thousands of smaller single unit molecules called nucleotides, all bound together. These molecules are the storehouse and delivery system of genetic t…
Spin, s, is the rotation of a particle on its axis, as the earth spins on its axis. The spin of a particle is also called intrinsic angular momentum. Angular momentum is momentum (mass times velocity) times the perpendicular lever arm (distance between point of rotation and application of force). An intrinsic property is one that depends on the essential nature of an object. The total angular mome…
Ribosomes are protein manufacturers within cells. Huge molecules of DNA, or deoxyribonucleic acid, coiled within the chromosomes of every living organism use a universal language called the genetic code. Employed by all cells in the same fashion, the information encoded in DNA acts as a set of instructions for the synthesis of vital protein molecules. Cells assemble thousands of different kinds of…
Shrews are small, mouse-like mammals of the family Soricidae, class Insectivora. They have large cutting, or incisor teeth, similar to those of a mouse. But unlike a mouse (which is a rodent and thus has teeth that continually grow), the teeth of shrews must last a lifetime. Also, their snout is narrower and more pointed than that of a mouse. There are more than 260 species of shrews. They vary up…
Rice is a species of grass (family Poaceae) that is an extremely important cereal crop. Two species of rice are grown as food: Oryza sativa and O. glaberrima. The natural range of both these rice species is tropical Asia, although rice can also be cultivated in warm-temperate regions. Of the two species, O. sativa is much more widely grown. In addition, there are seven major varieties of O. sativa…
Shrikes are 72 species of perching birds that make up the family Laniidae, in the order Passeriformes. The diversity of shrikes is greatest in Africa, with species also occurring in Europe, Asia, and Southeast Asia as far south as New Guinea. Two species occur in North America. Shrikes occur in a wide range of habitats, including forest edges, open forest, savanna, grassland, and some types of shr…
Shrimps are common, small invertebrates that occur in all marine ecosystems; in addition, some species have adapted to living in freshwater. All members of this group (class Crustacea, order Decapoda) are adapted for swimming. Most species, however, are bottom-dwelling animals that swim only occasionally. The body of most species of shrimps is compressed side-ways, or it may be more cylindrical in…
Spina bifida is a congenital neural tube defect caused by problems with the early development of the spinal cord. The main defect of spina bifida is the failure of closure of the vertebral column (the bony column surrounding the spinal cord) during embryogenesis. Embryogenesis refers to the stages of a developing embryo after fertilization of the egg by the sperm. Without closure of the neural tub…
Ricin is a highly toxic protein that is derived from the bean of the castor plant (Ricinus communis). The toxin causes cell death by inactivating ribosomes, which are responsible for protein synthesis. Ricin can be produced in a liquid, crystal or powdered forms and it can be inhaled, ingested, or injected. It causes fever, cough, weakness, abdominal pain, vomiting, diarrhea and dehydration and de…
The best known species is Spinacia oleracea. Two varieties are grown extensively, one with smooth leaves and another wrinkled, savoy variety. Both of these can be purchased fresh at produce stores. In the food packing industry, the smooth-leaved type is usually canned or frozen before shipping, and the savoy variety is packaged and shipped fresh. Spinach grows best in cool, temperate weather. Cool…
The spiny anteaters, or echidnas, make up five of the six species in the order Monotremata. These are primitive mammals that lay eggs like reptiles but have hair and suckle their young. One species of spiny anteater, Tachyglossus aculeatus, lives in Australia, Tasmania, and New Guinea. A second, T. setosus, is slightly larger and resides only in Tasmania. The other three species (in the genus Zagl…
The spiny eel, belonging to the order Notacanthiformes and the family Notacanthidae, is an eel-like fish that grows to more than 3.3 ft (1 m) long and lives in the north Atlantic Ocean. It has a series of short, thick spines on its back, and there are about 20 slender spines preceding its anal fin on its underside. This fish is a benthic fish, meaning that it lives close to or on the bottom of the…
Spiny-headed worms, or arrow worms as they are also known, belong to the phylum Chaetognatha. Their bodies are shaped like a torpedo with distinct head, trunk, and tail regions, the latter which bears a pair of finlike projections that probably assist with balance. Although many spiny-headed worms can swim, they usually conserve their energy and instead drift with the water current. The body is us…
Rickettsia are a group of bacteria that cause a number of serious human diseases, including the spotted fevers and typhus. Rod- or sphere-shaped, rickettsia lack both flagella (whip-like organs that allow bacteria to move) and pili (short, flagella-like projections that help bacteria adhere to host cells). Specific species of rickettsia include Rickettsia rickettsii, which causes the dangerous Roc…
A spiral is a curve formed by a point revolving around a fixed axis at an ever-increasing distance. It can be defined by a mathematical function which relates the distance of a point from its origin to the angle at which it is rotated. Some common spirals include the spiral of This famed spiral staircase in the Loretto Chapel (Chapel of Our Lady of Light) in Santa Fe, New Mexico, has no centr…
The spirometer is an instrument used in medicine to measure the volume of air inhaled and exhaled. The device is considered an essential tool in the detection of chronic obstructive pulmonary disease, which includes emphysema and chronic bronchitis. Chronic obstructive pulmonary disease was the fourth most common cause of death in the United States in 1993. In addition, spirometers are typically u…
Sickle cell anemia is an inherited blood disorder that arises from a single amino acid substitution in one of the component proteins of hemoglobin. The component protein, or globin, that contains the substitution is defective. Hemoglobin molecules constructed with such proteins have a tendency to stick to one another, forming strands of hemoglobin within the red blood cells. The cells that contain…
A river is a natural stream of freshwater with significant volume when compared to the volume of its smaller tributaries. Conveying surface water run-off on land, rivers are normally the main channels or largest tributaries of drainage systems. Typical rivers begin with a flow from headwater areas made up of small tributaries, such as springs. They then travel in meandering paths at various speeds…
Sieve of Eratosthenes is an almost mechanical procedure for separating out composite numbers and leaving the primes. It was invented by the Greek scientist and mathematician Eratosthenes who lived approximately 2,300 years ago. The natural numbers 1, 2, 3, 4,... can be classified into three groups: the prime numbers, which have no proper divisors other than 1; the composite numbers, which have two…
RNA, which is made up of nucleic acids, has a variety of functions in a cell and is found in many organisms including plants, animals, viruses, and bacteria. Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) differ functionally. DNA primarily serves as the storage material for genetic information. RNA can function as a carrier of genetic information, a catalyst of biochemical reactions, an ad…
RNA splicing is the process in which introns, or intervening sequences within a gene, are removed from ribonucleic acid (RNA) transcribed from deoxyribonucleic acid (DNA), prior to translation of RNA into protein. Prior to the early 1970s, the structure of genes had been elucidated and it was understood that genes were located with linear DNA sequences. The central dogma of molecular biology had b…
Split-brain functioning refers to how the two cerebral hemispheres of the brain are involved to different degrees in certain psychological and behavioral functions. In the normal brain the two hemispheres work together in a coordinated manner and these differences in functioning complement one another. The division of psychological and behavioral functions between the two cerebral hemispheres can …
Robins are songbirds in the family Musicicapidae, in the thrush subfamily, Turdinae, which contains more than 300 species, including various thrushes, chats, solitaires, redstarts, nightingale, wheatear, and others. The members of this family, known as robins, tend to have dark backs and reddish breasts. Except for this superficial resemblance, these robins are not particularly closely related, ot…
Sponges are the most primitive multicellular organisms that possess no proper organs. All members of this phylum (Porifera) are permanently attached to another surface, such as rocks, corals, or shells. More than 10,000 species have been described to date. Although some species occur in freshwater, the vast majority are marine, living mainly in shallow tropical waters. A wide range of forms occur …
Silicon is the chemical element of atomic number 14, symbol Si and atomic weight 28.085. In its crystalline form of dark gray crystals, it has a specific gravity of 2.42 at 68°F (20°C), a melting point of 2,588°F (1,420°C) and a boiling point 5,936°F (3,280°C). It exists also in an amorphous (shapeless) form, a brown powder. Silicon consists of three stabl…
Spontaneous generation, also called abiogenesis, is the belief that some living things can arise suddenly, from inanimate matter, without the need for a living progenitor to give them life. Through the centuries, the notion of spontaneous generation gave rise to a wide variety of exotic beliefs, such as that snakes could arise from horse hairs standing in stagnant water, mice from decomposing fodd…
In zoology, spores are structures that are used by organisms to survive a period of unfavorable environmental conditions, and can subsequently regenerate into the adult form once the environment again becomes favorable for growth. Depending on the species, spores are asexual, resting bodies, which can be one-celled or multi-cellular. Many protozoans have a stage in their life cycle that involves t…
Springtails are tiny insects in the order Collembola, a relatively ancient and primitive group in the sub-class of wingless insects known as Apterygota. Springtails have a fossil record extending back to the Devonian era, some 400 million years ago. Collembolans undergo complete metamorphosis, where the immature stages (nymphs) are tiny representations of the adult. Springtails are named for their…
The silk cotton family (Bombacaceae) is a group of about 200 species of tropical trees, some of which are of commercial importance as sources of lumber, fibrous material, or food. Species in the silk cotton family occur in all regions of tropical forest, but they are most diverse in Central and South America. …
Sinkholes are natural, circular depressions that form when water erodes easily dissolved or soluble rock located beneath the ground surface. Water moves along joints, or fractures, enlarging them to form a channel that drains sediment and water into the subsurface. As the rock erodes, materials above subside into the openings. Sinkholes range from a few feet (m) to several hundred ft (m) in width …
Skates are members of the class Chondrichthyes, the cartilaginous fish, the same class that contains sharks, rays, and chimeras. Skates, and their relatives the rays, comprise the order Rajiformes, which contains 318 species in 50 genera and 7 families. The skate family (Rajidae) is the largest family, encompassing about 120 species in 10 genera. The many species of skate vary greatly in size. The…
Spruces are species of trees in the genus Picea, family Pinaceae. The natural range of spruces is the Northern Hemisphere, where these trees occur in boreal and cool-temperate climates. These climates are common at high altitudes on the slopes of mountains and at high latitudes towards the north, south of the arctic tundra. Spruces sometimes dominate the forests in which they occur, or sometimes t…
Robotics is the science of designing and building machines that can be programmed to perform more than one function traditionally performed by humans. The word robot comes from a play written in 1920 by the Czech author Karel Capek. Capek's R.U.R. (for Rossum's Universal Robots) is the story of an inventor who creates humanlike machines designed to take over many forms of human work.…
Spurges or euphorbs are species of plants in the family Eurphorbiaceae. This is a rather large family of plants, consisting of about 7,500 species and 300 genera, mostly distributed in the tropics and subtropics, but also in the temperate zones. The most species-rich genera of spurges are the Euphorbia with about 1,600 species, and Croton with 750 species. Most species in the spurge family have a …
The square is used as the unit of area; that is, a figure's area is expressed as the number of equal squares of some standard, such as square inches or square meters, that the figure can contain. In Greek geometry, the area of a figure was determined by converting the figure to a square of the same area. This is easily accomplished for triangles, rectangles, and other polygons, but is often…
A skeleton is a sturdy framework of about 206 bones that protects the body's organs, supports the body, provides attachment points for muscles to enable body movement, functions as a storage site for minerals such as calcium and phosphorus, and produces blood cells. The skeletal system is a living, dynamic system, with networks of infiltrating blood vessels. Living mature bone is about 60% …
When n is a negative number, the square root √n is called imaginary. Customarily, √-1 is designated by i so that the square root of any negative number can be expressed as ai where a is a real number. Thus √-5 = 5 i. …
Squid is the common name for a group of marine mollusks (order Mollusca) with highly developed eyes and brain, and complex swimming behavior. About one-half the length (24-36 in; 60-90 cm) of the common North Atlantic species Loligo pealei consists of its streamlined cylindrical body, and the other half is its set of eight arms and two arm-like tentacles. These appendages are equipped with small s…
Squirrel fish, belonging to the order Beryciformes, are brightly colored, medium-sized fish that are active mostly at night. Squirrel fish live in rocky or coral reefs in tropical and warm temperate seas. Their most distinguishing characteristics are their large eyes and their ability to make sounds to ward off intruders. The order Beryciformes is composed of 15 families and about 150 species of m…
Skinks are smooth, shiny-scaled lizards in the family Scincidae, most of which occur in tropical and subtropical climates, although a few occur in the temperate zones. Most species of skinks occur in Africa, South and Southeast Asia, and Australia, with relatively few others occurring in Europe and North and South America. Their body is roughly cylindrical with distinctive overlapping scales on th…
Skuas comprise five species of sea birds in the family Stercorariidae, order Charadriiformes. These birds breed on the coastal tundra and barrens of the Arctic and Antarctic, and winter at sea and in coastal waters. Skuas are gull-like in many respects, with long, pointed wings, short legs, and webbed feet. However, skuas have a strongly hooked beak, elongated central tail feathers, and a generall…
Skunks are small North American mammals that share the carnivore family Mustelidae with weasels, otters, badgers, and the honey badger. They are distinguished from those other animals by their striking black and white color and their long-haired, fluffy tails. They are about the size of domestic cats. While many animals have anal glands that give off sharp odors, the skunks are the best known for …
The term rocket refers both to a non-air-breathing jet engine and to any vehicle it propels. Rocket fuels may be either solid or liquid. In the former case, the rocket is commonly known as a rocket engine, while in the latter case, it is usually called a rocket motor. A missile is an unmanned vehicle propelled through space, usually carrying some type of explosive intended to do harm to an enemy. …
Slash-and-burn is an agricultural system used in tropical countries, in which a forest is cut, the debris is burned, and the land is then used to grow crops. Slash-and-burn conversions are relatively stable and long-term in nature, and they are the leading cause of tropical deforestation. Usually, some type of slash-and-burn system is used when extensive areas of tropical forest are converted into…
Geologists define rocks as aggregates of minerals. Minerals are naturally occurring, inorganic substances with specific chemical compositions and structures. A rock can consist of many crystals of one mineral, or combinations of many minerals. Several exceptions, such as coal and obsidian, are not composed of minerals but are considered to be rocks. Common uses for rocks include building materials…
The squirrel family (Sciuridae) is a diverse group of about 50 genera of rodents, including the "true" or tree squirrels, as well as flying squirrels, ground squirrels, chipmunks, marmots, woodchuck, and prairie dogs. Members of the squirrel family occur in North and South America, Africa, Eurasia, and Southeast Asia, but not in Madagascar, New Guinea, Australia, or New Zealand. The …
A rodent is any mammal that belongs to the order Rodentia, which includes most mammals equipped with continuously growing incisor teeth that are remarkably efficient for gnawing on tough plant matter. The name rodent comes from the Latin word rodere meaning "to gnaw." Rodents live in virtually every habitat, often in close association with humans. This close association between roden…
Sleep is a state of physical inactivity and mental rest in which conscious awareness, thought, and voluntary movement cease and intermittent dreaming takes place. This natural and regular phenomenon essential to all living creatures normally happens with the eyes closed and is divided into two basic types: REM (rapid eye movement) and NREM (non-rapid eye movement) sleep. As passive as sleep appear…
Stalactites and stalagmites are speleothems formed by water dripping or flowing from fractures on the ceiling of a cave. In caves, stalagmites grow rather slowly (0.00028-0.0366 in/yr [0.007-0.929 mm/yr]), while in artificial tunnels and basements they grow much faster. Soda straw stalactites are the fastest growing (up to 40 mm/yr.), but most fragile stalactites in caves. Soda straw stalactites f…
Rollers are 16 species of terrestrial birds in the family Coraciidae. Rollers occur in Africa, Eurasia, and Australia. Most species are tropical, but some occur in temperate climates. Rollers are stout-bodied birds, ranging in body length from 9.5 to 13 in (24 to 33 cm). Most species have rounded wings, and a square or forked tail, although a few have elongated, decorative tail feathers. Rollers h…
The Standard Model is the complete catalogue of fundamental particles known to physicists at this time. It gives a complete account of the irreducible piece-parts or "fundamental particles" of which all matter and force are made, so far as those particles are known at this time. The Standard Model is not an all-embracing "theory of everything," however, because it does …
