HYDROSTATICS. 



The specific gravity of a body, either 

 fluid or solid, is ordinarily found by 

 means of the hydrostatic balance; a most 

 ingenious device for exactly ascertaining 

 the weight, either immersed in the wa- 

 ter, or in the air. The construction of 

 this instrument requires peculiar nicety, 

 but it may be appended to any common 

 balance; as will be understood from the 

 following description. Each scale should 

 have a small hook fixed to the centre of 

 its bottom, or lower side ; so these small 

 weights may be attached by means of 

 horse-hair, or fine silk, thence to suspend 

 a body in water without wetting the 

 scale. First weigh the body in the usu- 

 al manner in the scales, with great ex- 

 actness; immerse it in water, and the 

 equilibrium will be instantly destroyed. 

 To restore it, put into the scale, from 

 which the body immersed in the water is 

 suspended, as much weight as will bring 

 it even with the other scale, in which the 

 opposing weight remains unaltered. The 

 added weight will be equal to that of a 

 quantity of water equalling the immersed 

 body in bulk. Now, if the weight of the 

 body in air be divided by what it weighed 

 in the water, the quotient will show how 

 much that body is heavier than its bulk 

 of water. 



A guinea, new from the mint, will re- 

 quire 129 grains to be offered to its 

 weight in air; but, on being immersed in 

 water, will require 7i grains more to re- 

 store the equilibrium lost by the immer- 

 sion. From this we see, that a quantity of 

 water equal in bulk with the guinea 

 weighs 7J- grains, or 7.25, by which di- 

 vide 129, (the Weight in air,) and the 

 quotient will be 17.793; shewing that the 

 guinea is as 17-793, to one of water. 



But we sometimes have occasion to as- 

 certain the precise weight of bodies that 

 are lighter than water, say a piece of cork, 

 and which, if unaided, would float on its 

 surface. In such case, it is necessary to 

 affix a weight (having previously found 

 its exact poise) thereto ; when, by im- 

 mersing both, and deducting the amount 

 of the collateral weight, the residue will 

 be left to account of the cork. If you 

 would weigh quicksilver, it must be first 

 balanced in a glass bucket, of which the 

 weight is known, and which has been 

 weighed also by immersion. When the 

 bucket has been brought to equilibrium 

 in the water, pour in the quicksilver, and 

 the additional weight requisite to coun- 

 terbalance it will show its exact weight 



Perhaps the following general rules for 

 finding the specific gravity of bodies may 

 prove useful and familiar to every under- 



standing. I, "When the body is heavier 

 than water." Weigh it both in water, 

 and in the atmosphere, and the differ- 

 ence between the results will show the 

 quantity lost in the former mode ; then, 

 as the weight lost in water is to the 

 weight in air, so is the gravity of water 

 to the gravity of the body. 2. "When 

 the body, being specifically lighter, will 

 not sink in water." Render the body 

 heavy enough to sink by means of some 

 appendage, as a small piece of lead, &c. ; 

 weigh the body and the appendage, both 

 separately and together, in the air, and 

 in the water; find out how much each 

 loses in the water, and subtract those 

 losses from the whole weight of each in 

 air. Then, as the last remainder is to the 

 weight of the light body in air, so is the 

 gravity of water to the gravity of the bo- 

 dy. 3. "When a fluid is to be weighed." 

 Weigh the fluid in a cup, which is to be 

 deemed an appendage, and treated ac- 

 cording to the foregoing rule, observing, 

 that as the whole weight is to the loss of 

 weight, so is the gravity of the solid to 

 the gravity of the fluid. 



We may ascertain the respective 

 weights of two known ingredients in a 

 given compound thus : take the differen- 

 ces of every pair of the three specific 

 gravities; (viz. the specific gravities of 

 the compound, and of each ingredient :) 

 multiply each quantity by the difference 

 of the other two; then, as the greatest 

 product is to the whole weight of the 

 compound, so is each of the other two 

 products to each respective weight of the 

 two ingredients. 



If a piece of glass, or of metal, be im- 

 mersed by suspension in different fluids, 

 it will lose in weight; that is, it will re- 

 quire an equipoise, according to the 

 weight of the fluids respectively : observ- 

 ing, that in the lightest fluid, say alcohol, 

 it will lose least weight. This is the prin- 

 ciple on which the hydrometer acts, as 

 will be subsequently shown. 



Vessels filled with water weigh more 

 than when empty : to prove this, let a 

 bottle be loaded so as to sink in a pail 

 of water, deep enough for the water to 

 cover its mouth, which should be pre- 

 viously closed by a plug, in such man- 

 ner as might be easily pushed in ; ap- 

 pend the bottle in equilibrio to the hy- 

 drostatic balance, and drive in the plug : 

 the water will follow and destroy the 

 equilibrium. 



Fluids press every away alike, though 

 their general tendency is to gravitation. 

 Thus, if a vessel be made weaker in the 

 side than at the bottom, and be so laden 



