GRE 



GRO 



and 2, inversely as the squares of their 

 distances. Thus, if the mass of one body 

 be 2, 3, 4, or 5 times greater than that of 

 another, its gravity is also 2, 3, 4, or 5 

 times greater; and with a distance of 

 2, 3, 4, or 5 times greater, the force of 

 gravity will be 4, 9, 16, or 25 times less. 



GRAVITY (gravitas, heaviness). A 

 name given, in Physics, to the tendency 

 which all bodies have to the centre of 

 the earth. The term is often used syno- 

 nymously with attraction, from which, 

 however, it differs as a species from a 

 genus : we speak, for instance, of capil- 

 lary attraction and of magnetic attrac- 

 tion, but not of capillary or magnetic 

 gravity. 



1. Gravity, specific. The density of 

 bodies, as ascertained by comparison with 

 an equal bulk of water, which is assumed 

 as unity, or the absolute weight. One 

 substance is said to have a greater specific 

 gravity than another, when a less portion 

 of its bulk is of equal weight to that other. 

 Thus, a cubic inch of platinum is nearly 

 twice as heavy as a cubic inch of silver ; 

 the specific gravity of the former is, 

 therefore, double that of the latter. 



2. Gravity, centre of. That point in a 

 body, about which all its parts, in any 

 situation, balance one another. Hence 

 if the body could be suspended from this 

 point, it would remain at rest, with re- 

 ference to its tendency towards the earth, 

 in whatever respective position the sur- 

 rounding parts may be turned. Thus, 

 the centre of gravity of a globe, if of uni- 

 form density, is its common centre, and 

 that of a balanced beam is the pivot on 

 which it turns. 



3. Gravity, line of direction of. That 

 line which passes through the centre of 

 gravity of a body in a direction to the 

 centre of the earth. 



GRAYSTONE. A term suggested for 

 designating certain volcanic rocks com- 

 posed of felspar, augite, or hornblende, 

 and iron; the felspar being sometimes 

 replaced by leucite or melilite. 



GREEK FIRE. An explosive sub- 

 stance, invented by the Greeks, supposed 

 to consist of asphaltum, nitre, and sul- 

 phur. 



GREEN EARTH. An earthy or granu- 

 lar variety of chlorite, which occurs mas- 

 sive, dispersed in rocks, or enclosed in 

 some simple minerals, as rock-crystal, 

 and often in the globular cavities of cer- 

 tain rocks. It is the mountain green of 

 artists in water colours. 



GREEN MINERAL. A carbonate of 

 156 



copper, employed as a pigment. Scheele'a 

 green is an arsenite of copper; Bruns- 

 wick green, an oxychloride of copper. 



GREENSAND. A designation of beds 

 of sand, sandstone, and limestone, be- 

 longing to the Cretaceous Period. They 

 are so named from the abundance of 

 green earth or chlorite which is often 

 scattered through their substance. 



GREENSTONE. A variety of trap, 

 consisting of hornblende and felspar, in 

 nearly equal proportions, and having a 

 granitic structure. The name is not very 

 appropriate, for, besides being sometimes 

 greenish, it is not uncommonly reddish 

 or whitish, the colour depending chiefly 

 on that of the felspar, which is generally 

 compact, or at least not highly crystalline. 

 This rock is called diabase by the French 

 geologists, who name the compact green- 

 stone aphanite. 



GREGORIAN TELESCOPE. A re- 

 flecting telescope, having a hole in the 

 centre of the great speculum, through 

 which the image is thrown by the 

 small reflector to the eye. The dis- 

 tinctness of the object seen through 

 this telescope is somewhat diminished 

 by the hole in the great speculum. 



GRENATITE. Staurotide. A desig- 

 nation of the prismatic garnet. 



GREYWACKE' {grau, grey, wache, a 

 provincial miner's term). A German 

 designation of some of the most common 

 fossiliferous strata, consisting of a series 

 of arenaceous and slaty rocks, of evidently 

 mechanical origin, intermixed with small 

 beds of limestone. The rocks are very 

 often of a grey colour. 



GRIDIRON PENDULUM. Harri- 

 son's compensation pendulum, which is 

 constructed of different substances, so 

 combined that the effects of heat coun- 

 teract each other, and the length of the 

 rod remains unaltered. 



GRIT. A provincial name for a coarse- 

 grained sandstone, employed for mill- 

 stones, &c. Calcareous grit, or sandstone, 

 is one of the subdivisions of the Middle 

 Oolite Formation. 



GROSSIFICA'TION. A term applied 

 to the swelling of the ovary of plants 

 after fertilization. 



GRO'SSULAIRE. A green crystal- 

 lized mineral, found imbedded with ve- 

 suvian, in a pale greenish-grey claystone 

 in Siberia. 



GROTTO DEL CANE. The Dog'a 

 Grotto ; a cave in Italy, in which there is 

 a constant natural exhalation of carbonic 

 acid gas, which, occupying the lowest 



