JACK

in practical mechanics, portable hand-operated device for raising heavy weights through short distances, exerting great pressures, or holding assembled work firmly in position, as in jacking up a building to prevent settling or keeping it in position while replacing a foundation. The jack's effectiveness stems from the ratio of the load, or weight of the object raised, to the amount of force applied to the handle of the jack. This ratio can be made quite high by the use of a gear or screw to regulate the upward extension of the jack. A ratchet allows a heavy weight to be raised in short successive stages, with the jack locking whenever force is not being applied to its handle. Though limited in capacity by the requirements of portability and ease of manual operation, a jack may be built to lift, or exert a force of, several tons. A familiar example of this device is the automobile jack, which is used to raise one end of a car to permit the changing of a tire. any of numerous species of fishes belonging to the family Carangidae (order Perciformes). The name jack is also applied collectively to the family. Representatives can be found in temperate and tropical portions of the Atlantic, Pacific, and Indian oceans and occasionally in fresh or brackish water. Although body size and shape vary greatly among jacks, many of the more than 150 species are characterized by laterally compressed bodies, rows of large, spiky scales along the side near the tailfin, small scales resulting in a smooth appearance, and a deeply forked tail. Many have a bluish green, silvery, or yellowish sheen on the body. Jacks are important commercially and are favoured sports fishes. Some of the most popular marine game fishes are the amberjacks (genus Seriola), which are found worldwide. The greater amberjack (S. dumerili) of the tropical Atlantic is one of the largest members of the jack family, often attaining lengths of 1.8 m (6 feet). The genus Caranx includes several species of smaller but popular game fish, such as the crevalle jack (C. hippos) of warm Atlantic waters and the yellow jack (C. bartholomaei), which frequents warm Atlantic waters and is noted for its golden-yellow sides and fins.