GRAVITY. 



The moon, therefore, gravitates to- 

 wards the earth, and reciprocally the 

 earth towards the moon, and this is also 

 farther confirmed by the phenomena of 

 the tides. 



The like reasoning 1 may also be applied 

 to the other planets. For as the revolu- 

 tions of the primary planets round the 

 sun, and those of the satellites of Jupiter 

 and Saturn round their primaries, are 

 phenomena of the same kind with the re- 

 volution of the moon about the earth ; 

 and as the centripetal powers of the pri- 

 mary are directed towards the centre of 

 the sun, and those of the satellites to- 

 wards the centres of their primaries ; 

 and, lastly, as all these powers are reci- 

 procally as the squares of the distances 

 from the centres, it may safely be con- 

 cluded that the power and causes are the 

 same in all. Therefore, as the moon gra- 

 vitates towards the earth, and the earth 

 towards the moon, so do all the seconda- 

 ries to their primaries, and these to the 

 secondaries ; and so also do the primaries 

 to the sun, and the sun to the primaries. 

 Newton's Princip. lib. iii, prop. 4, 5, 6; 

 Greg. Astron. lib. i. sect. 7, prop. 46 and 

 47. 



The laws of universal gravity are the 

 same as those of bodies gravitating to- 

 wards the earth, before laid down. See 

 ASTRONOMY, ATTRACTION, GEOGRAPHY. 



GRAVITY, specific. Boyle is araong the 

 first of our philosophers, who suggested 

 the advantage that chemistry and minera- 

 logy might derive from an attention to 

 the specific gravities of bodies. Much 

 advantage may indeed be derived from 

 this property in the general determina- 

 tion of the classes of minerals, and the 

 purity of some metallic bodies ; and it is 

 very probable, that an attention to the 

 specific gravities, capacities for heat, fu- 

 sibilities, volatilities, laws of crystalliza- 

 tion, elasticity, hardness, tenacity, mallea- 

 bility, and some other obvious specific 

 properties of bodies, may produce a more 

 intimate acquaintance with the mutual ac- 

 tions of their particles, than any we have 

 hitherto acquired. 



Annexed to this article is a table of spe- 

 cific gravities, from various authors. It 

 appeared useless to carry it to more than 

 four places of figures, as the temperatures 

 are not noted ; and the various specimens 

 of the same substance often differ in the 

 third figure. Besides this, it is remarked 



by Nicholson, in his " Chemical Diction- 

 ary," that the fifth figure changes in wa- 

 terat every three degrees of Fahrenheit's 

 thermometer; that lead, tin, and proba- 

 bly all other metals, though cast out of 

 the same fusion, will vary in their speci- 

 fic gravities in the third figure, from cir- 

 cumstances not yet determined, but most 

 likely from the cooling, as is seen in 

 the hardening of steel ; that salts, and 

 other artificial preparations, retain more 

 or less of the solvent they were separat- 

 ed from, according to the temperature at 

 which the crystallization was effected; 

 and that all parts of organized substances 

 not only differ, according to the place of 

 their production, their age, and other cir- 

 cumstances, but likewise from their dry- 

 ness, moisture, and manner of preserva- 

 tion. 



The specific gravity of solids is deter- 

 mined by weighing them, first in air, and 

 then in water. The loss of weight, aris- 

 ing from the action of the water, is equal 

 to that of a mass of the fluid possessing 

 the same dimensions as the solid itself! 

 Whence it is easy to construct a general 

 table of specific gravities, by reducing 

 the proportion of the absolute weight to 

 the loss sustained by immersion, into 

 terms of which that expressing water 

 shall be unity. If the solid be so light as 

 to float upon water, it is convenient to at- 

 tach to it a heavier body sufficient to cause 

 it to sink, but the weight of which in wa- 

 ter must be added in computing the loss. 

 The specific gravity of fluids is ascertain- 

 ed by weighing a known body immersed 

 in them. For the loss by immersion will 

 accurately show the weight of the same 

 bulk of the fluid ; and, consequently, the 

 proportion of these several quantities to 

 the loss the same solid sustained in water 

 being reduced, as in the other case, to the 

 common standard of unity, will exhibit 

 the specific gravity. Other methods are 

 likewise used in experiments with fluids. 

 Thus equal bulks of different fluids may 

 be weighed by filling a small bottle with 

 a ground stopper with each respectively, 

 and from their several weights the weight 

 of the bottle and stopper must be deduct- 

 ed. Or, otherwise, the instrument called 

 the hydrometer may be used. See HY- 

 DROMETER. This possesses the advantage 

 of portability, speed, and a degree of ac- 

 curacy, not easily obtained by the use of 

 ordinary balances. 



