OCR Text |
Show 114 SPECIFIC GRAVITY. same gravitation, in proportion to its quantity of matter, as the lead which sinks to the bottom. The absolute gravity of any one substance is exactly the same as that of any other; and the only variations are, there being less or more of the substance in an equal space, or substances being nearer to each other or farther asunder. The variations of weight in an equal bulk, or of bulk in an equal weight, of different kinds of matter,. are called their specific gravities, because they ar~ one of the means by which the species of matter ar<. known and distinguished from each other. The specific gravity is known by weighing equal bulks, or measuring equal weights; and that which measures least when the weights are equaJ, and weighs most when their bulks are equal, has the greater specific gravity. It is of no consequence to which of these methods singly we have recourse, or wlwther we have recourse to both of them jointly -as we must have in all cases where the speci·mens which we compare are neither of the same weight nor of the same bulk. In those cases ihe common mode of ~xpress~on is, that the specific g:ravity is! as the we1ghts duectly, and as the bulks mversely -which means, that if both substances are weighed, and both measured, by standards which a:re the same as applied to both, then, if the weight of each be multiplied by the bulk of the other, the products wili express the relation of the specific gravities. Eut the specific gravity of particular substances is not, like the absolute gravity of one substance, unalterable. The very :notion of it is compound : both weight and measure enter into it, and its value is expressed by their product ; and the same product may be obtained from any two numbers, if the one be increased in the same proportion as the other is diminished. Thus the number sixteen is four times four, or two times eight, or one and one-third times twelve, or one time sixteen, or one-half time thirty. VARIATIONS OF GRAV11Y. 115 two, or one-quarter time sixty-four-or, in short, some product of any number that can possiblv be named. . There ~re various natural causes that alter the .speci~c gravity of substances, though more extensn: ely m some than in others ; and some of the most Important, as .well as the most curious results and appearances m nature are owing to those cha1~ges.. In dry woo_d, the changes of specific ~av1ty, m the same piece, are very small; while, In t~e comm~n air that we breathe, they are great; and m c~untries and at seasons that have the weather varia?le, they are constantly taking place. They are, mdeed, among the immediate causes of ~orne of the changes in the weather, and they are In other cases the effects-on that subject it is not very easy to distinguish between causes and effects. In the events themselves, there is no difference between what we call causes, and what we call effects ; for e\·ery cause is the effect of a former cause, whether that cause be known to us or not. and every effect is the cause of a future effect' wheth~r or not we shall discover or otherwise kno~ that effect. The only other variation of gravity which it is necessary to men~io~?-, for J?UrJ?oses so very general as o~rs, Is the variation of Its mtensity with change ?f distance. The farther any one piece of matter · IS remov~d from any other piece, the less does the one gravitate towards the other. If, for instance one bodY: be re~ov~d to twice the distance fro~ another, Its gray1tatmg tendency will be only onefourth of. what .1t was befor~; and i.f it be brought t? on~-thud of Its former distance, It will gravitate nme .hmes as much. It is usual to say that bodies gravit~te t<;>wards each other inversely as the squares o~ their dtstances,-that is, if one body is at the distance two, a.nd another at the distance three, the bod~ at tw? Will, on account of distance, gravitate as nme, while the body at three, gravitates as four .. |