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LEFT GBAUDENZ 384 GRAY GBAUDENZ (grou'dents), a town in the province of West Prussia, Germany; on the Vistula, 37 miles N. of Thorn. It carries on a trade in corn, wool, and cattle, and has iron foundries, brew- eries, and tapestry and cigar manufac- tories. About a mile N. of it on a hill is the fortress of Graudenz, built in 1776, and successfully defended against the French in 1807. It was maintained as a fortress till 1874, and now serves as a barrack and military prison. GRAVEL, small pebbles, stones, ov fragments of stone, intermixed with sand loam, clay, flints, etc., formed by the action of water upon disintegrated portions of rock. In pathology, the presence of minute concretions in the urine. It is usually owing to the presence of uric acid, urates, oxalates, and phosphates. Am.ong exceptional urinary calculi are carbonate of lime, cystine, xanthine, fatty and fibrinous concretions. The chief symp- toms are dull, aching pains over the renal regions, extending to the thighs. GRAVELINES (grav-len'), a town in the department of Nord, France; at the mouth of the Aa, 13 miles E. N. E. of Calais. Considerable historical interest is attached to the place, as the scene of Egmont's victory over the French (1558), and the place off which the English dis- persed the Armada (1588). It was taken by the French in 1644, retaken by the Austrians after a 10 weeks' siege in 1652, and finally recaptured in 1658 by Louis XIV., who had it fortified by Vau- ban. Pop. about 6,000. GRAVELOTTE (grav-lof), a village of Lorraine, 7 miles W. of Metz. There, on Aug. 18^ 1870, the French under Bazaine sustained a severe defeat by the Germans. GRAVESEND, a port and borough of Kent, England, on the Thames, 24 miles E. S. E. of London. It consists of the old town, with narrow, irregular streets, and of the handsome new town on the high ground. In the vicinity are exten- sive m.grket gardens; and many of the inhabitants are employed in fishing. Gravesend forms the limit of the port of London; and here pilots and custom- house officers are taken on board of ves- sels going up the river. It carries on some shipbuilding, iron-founding, soap- making, and brewing, and a consider- able trade in supplying ships' stores. Gravesend was incorporated under Eliza- beth; was originally a hythe, or landing- place; and is mentioned as such in Domesday. Here the fleets of early voy- agers, as that of Sebastian Cabot in 1553, and of Martin Frobisher in 1576, assembled, and here the lord mayor; aldermen, and city companies of London were wont to receive all strangers of eminence and to conduct them up the river in state. Pop. about 30,000. GRAVITY, in physics, the terrestrial gravitation, the operation of the law of gravitation on the earth, specially in making heavy bodies fall in all parts of the planet in the direction of its center. Newton and Bessel have shown that in a vacuum a sovereign and a feather will fall with equal speed, though the rate will be very different in the atmospheric air. The attraction of the whole earth, considered as a sphere, on a body at its surface, is the same as if the whole matter of the earth were collected at its center. The attraction of the earth on a body within its surface is the same as if the spherical shell situated between the body and the earth's surface was re- moved; or is the same as if all the matter situated nearer to the earth's surface than the body was collected at the center, and all the matter situated at a greater distance was removed. The weight of a body is proportioned to the attraction which it exerts, hence gravity in many cases means simply weight. GRAVITY, CENTER OF. See CENTER OF Gravity, under Center. GRAVITY, SPECIFIC, in physics, the relative density of a substance; the weight of a body compared with that of another body having the same magni- tude. To obtain this, it is first weighed in air, which shows its absolute weight. Next it is weighed in water, to show how^ much it loses in this element. There have now been ascertained the absolute weights of two bodies of equal bulk — viz., the one experimented on, and water, and the ratio of these weights is that also of their specific gravities. Let 1 be the weight of water, and first let the body be heavier than that liquid, then the weight which it loses in water is to the absolute weight as 1 to the specific grav- ity required. If lighter than water, then as the weight of the body in air, plus the weight needful to make it sink in water, is to its weight in air, so is 1 to the specific gravity. On this prin- ciple are constructed such instruments as Nicholson's portable balance. In solids and liquids the standard is generally dis- tilled water; for the gases, atmospheric air. Specific gravity is proportionate to density, and the words may be used al- most interchangeably. GRAY, a town in the department of Haute-Saone, France, on the Saone, here crossed by a stone bridge of the 13th cen-