SPECIFIC GRAVITY OP BODIES. 



LESSON 24, \ 



Specific Gravity of Bodies. j 



By the specific gravities of bodies we mean the relative \ 

 weights, which equal bulks of different bodies have to each ' 

 other. And it is usual to compare them with that of water, 1 

 as it is by weighing bodies in water that their specific gra- ] 

 vities are found. A body immersed in a fluid will sink to \ 

 the bettom, if it be heavier than its bulk of fluid; if it be 1 

 suspended therein, it will lose as muchof what it weighed in ^ 

 air, as its bulk of the fluid weighs. The instrument gene»- \ 

 rally used for obtaining the specific gravities is called the \ 

 hydrostatical balance ; it does not differ much from the com- \ 

 mon balance. The general rule for finding the specific gra- | 

 vity of a solid, heavier than water, as a piece of metal, is -^ 

 this : weigh the body first in air, in the usual way, then 

 weigh it when it is plunged in water, and observe how much \ 

 it loses of its weight in this fluid, and dividing the former.^ 

 weight by the loss sustained, the quotient is the specific gra- : 

 vity of the body, compared with that of water. As an ex- 1 

 ample, it is usual to take a guinea, which weighs in air one  

 hundred and twenty-nine grains, and when suspended by \ 

 means of a fine hair, and immersed in water, it is found to ; 

 balance one hundred and twenty-one grains and three-quar- \ 

 ters, losing of its weight seven grains and a quarter ; now \ 

 one hundred and twenty-nine divided by seven and a quar- - 

 ter, gives about seventeen for the quotient ; that is, the spe- i 

 cific gravity of a guinea compared with that of water, is as ' 

 about seventeen to one. And thus, any piece of gold may \ 

 be tried, by weighing it first in air, and then in water ; and i 

 if^ upon dividing the weight in air, by the loss in water, the \ 

 quotient comes to be about seventeen, the gold is good ; if ] 

 the quotient be eighteen, or between eighteen and nineteen, | 

 the gold is very fine ; but if it be l.-ss than seventeen, the ^ 

 gold is too much alloyed with some other metal. The same ; 

 principle is universal. Hence we soe the reason why boats ] 

 or other vessels float on water ; they sink just so low, that j 

 the weight of the vessel, with its contents, is equal to the \ 

 quantity of water which it displaces. The method of ascer- ; 

 gaining the specific gravities of bodies, was discovered by j 



