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* GRAVESANDE. appointed professor at the Universitj' of Leyden, and in that capacity had mudi inllueiice upon the development of natural science. He invented the first heliostat, and was one of the first scien- tists to accept Nerton's theory of gravitation. His principal productions include: Physices Ele- menta M at Iieuiatica Experimentis Confirmata sive Introductio ml Philosophiam Newtoiiidimm (3d edi 1742) and Philosophice Xewloitiaiiia; Institu- tioncs (2d ed. 1744). GRAVES'S DISEASE. See Basedow's Dis- ease. GRAVESEND, gravz'end. A market-town, numici|ial borough, and river port in Kent. Eng- land, 30 miles below London, on the right bank of the Thames (Map: England, G 5). Its pros- perity has always depended upon London, of which it is the boundary port, where pilots and revenue oflicers board vessels ascending the river, and where troops and passengers embark for long voyages. Its extensive fruit and vege- table gardens form the chief supply of the London markets; it carries on an important trade in fish and in. ships' supplies, has boat-building, iron-founding, brewing, and soap-boiling. The town occupies a commanding position on the first rising ground after entering the river, and ex- tends for two miles along the water-front, the older and lower portion with narrow streets and the upper and newer portion with handsome thor- oughfares, squares, and terraces. Its salubrious air and beautiful scenery make it a favorite ex- cursion and summer resort. Its public buildings are conspicuous, and include the late Decorated Milton parish church of the time of Edward II., and the Gravesend parish church, reerected in 1731, which was the burial-place of the celebrated Indian princess Pocahcmtas in 1616. Among the educational establishments are the free grammar school (founded 1580), Parrock Hall Industrial School. Jlilton Mount Congregational School for Ministers' Daughters, science and art schools, and a free library. The Doniesdati Bool: mentions Gravesend as a hythe or landing-place. A town soon grejv up after the Conquest, and the hythe was the as- sembling-place for early navigators, including Sebastian Cabot and Martin Frobisher. Here the London Lord IMayor,' aldermen, and city companies received all eminent foreigners and conducted them up the river in stately pro- cession. The privileges granted by Richard IT. and Henry- IV. were confirmed by Queen Eliza- beth's charter of incorporation in 1:573. The fortified Vaubanian earthworks, constructed in the reign of Charles II. to reenforce Tilbury Fort, have been strengthened in recent years at considerable expense. The municipal borough comprises the parishes of Gravesend and Milton. Population, in 1891, 24,000: in 1901, 27.200. GRAVIER, gra'yya', Jacques (?-1708). A French missionary in Canada and Illinois. He continued the work of Marquette among the In- dians for several years, but was constantly op- posed by the medicine men. Be was very success- iful. however, among the Kaskaskia Indians, many of whom he baptized. Dangerously wounded by a hostile native, he sailed for Europe in 1706, re- turned in 1708. only partially cured, and died at Jlobile soon after. He wrote three works on the Indian missions and Louisiana afTairs, and compiled a grammar of the Illinois tongue. 156 GRAVITATION. GRAVINA IN PUGLIA, gni-ve'na en poo'- lya. A city on the Gravina, in the Province of Bari delle Puglie, Italy, 89 miles southeast of Foggia (Map: Italy, L 7)). It has a collegiate church, an episcopal library, a hospital, and sev- eral asylums. It markets grain, vegetables, wool, cheese, wine, horses, uuiles, oxen, and sheep. The annual fair in April is important. It was a favorite hunting-place of Emjieror Frederick II., who built the castle. Population of commune, in 1881, 16.905; in 1901, 18,685. GRAVING DOCKS. See Docks. GRAVITATION ( Fr. gravitation, from Fr. graviter, to gravitate, from Lat. gravitas, weight, from grai-is, hea'y). The idea that there is an action between the earth and its moon and the sun and its planets jjerfectly analogous to that between the earth and a falling body occurred to many astronomers and students of science jjre- vious to the announcement by Newton in 1687 of his famous law of universal gravitation; but it was reserved for Xewton to give this law exact mathematical exjjression. All bodies, when raised into the air and left unsupported, fall to the earth. The force which causes them to do so is termed gravity, and. universal experience shows, acts toward the earth's centre; more strictly, it acts in a direction perpendicular to the surface of still water. But if a body, such as a stone, be projected obliquely into the air, it describes a curved path, and when it meets the earth in its descent, its direction is not toward the centre, but inclined to it at the angle of projection. (See Prciectiles. ) Observing this, and that the body, if not stopped Ijy the earth's surface, would continue to move in a curve, it is easy to imagine that it might circulate round the earth's centre as the moon does round the earth. (See Central Forces.) Obser'ing now the time of revolution of the moon, we can calculate the force with which it tends to leave the path (centrifu- gal force). This must be balanced by an equal attractive force, or we should lose the moon. But then the question arises, Is this attractive force the same as the force acting on bodies near the surface of the earth ? The answer is that it is a force 3600 times less energetic. If. then, gravity be the force which really holds the moon to its path, we must explain why it acts upon it so much more feebly than it would were it a body on the earth's surface. The explanation is given at once if we suppose gravity to be a force whose magnitude diminishes with increase of distance, and inversely as the squares of the distances at which it is exerted : for the distance of the moon from the earth's centre is about 60 times that of the earth's surface from its centre, and 3600: 1 : ; 60=: 1. We infer that this explanation is correct from the fact that there is nothing inadmissible in such a diminution of force with increase of distance, and in tlie- argument drawn from the necessity of otherwise supposing some other force tlian gravity to be employed in de- flecting the moon, and the force of gravity to cease at some unknown level. On these views Newton is understood to have at first rested his law of universal gravitation. Every particle of matter in the universe attracts every other particle with a force directly proportional to the mass of the attracting particle, and inversely to the square of the distance between them. New- ton, before conceiving the law, had explained the