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 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…
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…
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…
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 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…
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…
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 …
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…
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…
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…
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…
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…
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…
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…
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…
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…
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…
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…
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% …
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. …
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 …
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…
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…
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…
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…
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…
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…
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…
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 …
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…