Sewa \'sc-wn\ River River, Sierra Leone. Formed by the junction of the Bagbe and Bafi rivers, it flows 150 mi (240 km) to join the Waanje River and form the Kittam, which empties into the Atlantic Ocean. The coun¬ try’s most important commercial river, it is extensively panned for dia¬ monds.
sewage system Collection of pipes and mains, treatment works, and discharge lines (sewers) for the wastewater of a community. Early civi¬ lizations often built drainage systems in urban areas to handle storm run¬ off. The Romans constructed elaborate systems that also drained wastewater from the public baths. In the Middle Ages these systems fell into disrepair. As the populations of cities grew, disastrous epidemics of cholera and typhoid fever broke out, the result of ineffective separation of sewage and drinking water. In the mid-19th century the first steps were taken to treat wastewater. The concentration of population and the addi¬ tion to sewage of manufacturing waste that occurred during the Industrial
Sevres soft-paste porcelain jardiniere decorated with flowers and cupids painted in reserves on rose Pompadour ground, 1761; in the Victoria and Albert Museum, London
COURTESY OF THE VICTORIA AND ALBERT MUSEUM, LONDON; PHOTOGRAPH, WILFRID WALTER-EB INC.
© 2006 Encyclopaedia Britannica, Inc.
1728 I Sewall ► sexual response
Revolution increased the need for effective sewage treatment. Sewer pipe is laid following street patterns, and access holes with metal covers allow periodic inspection and cleaning. Catch basins at street corners and along street gutters collect surface runoff of storm water and direct it to the storm sewers. Civil engineers determine the volume of sewage likely, the route of the system, and the slope of the pipe to ensure an even flow by grav¬ ity that will not leave solids behind. In flat regions, pumping stations are sometimes needed. Modem sewage systems include domestic and indus¬ trial sewers and storm sewers. Sewage treatment plants remove organic matter from waste water through a series of steps. As sewage enters the plant, large objects (such as wood and gravel) are screened out; grit and sand are then removed by settling or screening with finer mesh. The remaining sewage passes into primary sedimentation tanks where sus¬ pended solids (sludge) settle out. The remaining sewage is aerated and mixed with microorganisms to decompose organic matter. A secondary sedimentation tank allows any remaining solids to settle out; the remain¬ ing liquid effluent is discharged into a body of water. Sludge from the sedimentation tanks may be disposed of in landfills, dumped at sea, used as fertilizer, or decomposed further in heated tanks (digestion tanks) to produce methane gas to power the treatment plant.
Sewall Vsii-olX, Samuel (b. March 28, 1652, Bishopstoke, Hampshire, Eng.—d. Jan. 1, 1730, Boston, Mass.) British-American colonial mer¬ chant and jurist. He immigrated to America as a boy and became man¬ ager of the New England colonial printing press (1681-84) and a member of the governor’s council (1684-1725). In 1692 he was appointed to pre¬ side at the Salem witch trials, in which 19 people were executed. Later admitting the error of the court’s decision, he stood silently in the Old South Church in Boston in 1697 while his confession of error and guilt was read aloud. His three-volume Diary (published 1878-82) provides a view of New England Puritan life.
Seward Vsu-ordV William H(enry) (b. May 16, 1801, Florida, N.Y., U.S.—d. Oct. 10, 1872, Auburn, N.Y.) U.S. politician. He served in the New York state senate (1830-34) and as governor (1839-43). In the U.S. Senate (1849-61), he was an antislavery leader in the Whig and Repub¬ lican parties. A close adviser to Pres. Abraham Lincoln, he served as U.S. secretary of state (1861-69). He helped prevent foreign recognition of the Confederacy and obtained settlement in the Trent Affair. In 1865 he was stabbed by a conspirator of John Wilkes Booth but recovered. He is best remembered for successfully negotiating the Alaska Purchase (1867), which critics called Seward’s Folly.
Seward Vsii-ordX Peninsula Peninsula, western Alaska, U.S. Its tip, Cape Prince of Wales, on the Bering Strait, is the most westerly point of North America. The peninsula is about 180 mi (290 km) long and 130 mi (209 km) wide; its highest peak is 4,720 ft (1,439 m), in the Kigluaik Mountains. The city of Nome is on its southern coast.
Sewell \'sii-3l\, Anna (b. March 30, 1820, Yarmouth, Norfolk, Eng.—d. April 25, 1878, Old Catton, Norfolk) British writer. She was introduced to writing by her mother, an author of juvenile best-sellers, and her concern for the humane treatment of horses began early in life. Confined to her house as an invalid, she spent her last years writing the children’s classic Black Beauty (1877), a fictional autobiography of a gentle, highbred horse. It had a strong moral purpose and is said to have been instrumental in abolishing the cruel practice of using the checkrein (a short rein used to prevent a horse from lowering its head).
sewing machine Machine for stitching material (such as cloth or leather), usually having a needle and shuttle to carry thread and powered by treadle or electricity. Invented by Elias Howe in 1846 and successfully manufactured by Howe and Isaac Merritt Singer, it became the first widely distributed mechanical home appliance and has also been an important industrial machine. Modern sewing machines are usually powered by an electric motor, but the foot-treadle machine is still in wide use in much of the world.
sex Sum of features by which a member of a plant or animal species can be placed into one of two complementary reproductive groups, male or female. In both plants and animals, sex is determined by the reproductive cells (gametes) produced by the organism. The male produces sperm cells, and the female produces EGG cells. Males and females may or may not have apparent structural differences, but they always have functional, hormonal, and chromosomal differences. Patterns of behaviour, some¬ times elaborate, may also distinguish the sexes in some species. See also REPRODUCTIVE BEHAVIOUR.
sex chromosome Either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of mam¬ mals are designated X and Y; in humans, they constitute one pair of the total 23 pairs of chromosomes. Individuals possessing two X chromo¬ somes (XX) are female; those having one X and one Y chromosome (XY) are male. The X chromosome is larger and carries more genetic informa¬ tion than the Y. Traits controlled only by genes found on the X chromo¬ some (e.g., hemophilia, red-green colour blindness) are said to be sex- linked. Sex-linked traits occur far more frequently in males than in females, since a male inheriting an allele for a recessive (see recessiveness) trait on the X chromosome lacks a corresponding allele on the Y chro¬ mosome that might counteract its effects. Several disorders are associated with an abnormal number of sex chromosomes, including Turner syndrome and Klinefelter syndrome.
sex linkage See linkage group
sextant Instrument for determining the angle between the horizon and a celestial body—such as the Sun, the Moon, or a star—used in celestial navigation to determine latitude and longitude. It consists of a metal arc, marked in degrees, and a movable radial arm pivoted at the centre of the arc’s circle. A telescope, mounted rigidly to the framework, is lined up with the horizon. The radial arm, on which a mirror is mounted, is moved until the star is reflected into a half-silvered mirror in line with the tele¬ scope and appears, through the telescope, to coincide with the horizon. The angular distance of the star above the horizon is then read from the graduated arc of the sextant. From this angle, the latitude can be deter¬ mined (within a few hundred metres) by means of published tables, and by consulting an accurate chronometer the longitude can be established. Invented in 1731, the sextant replaced the octant and became an essen¬ tial tool of navigation.