310 Hydrostatics. 



and that of the body immersed in it, by S', S, respectively, and 

 the bulk of the body by b, we shall have, Sb S' 6 for the 

 excess of the weight of the body above that of an equal bulk of 

 the fluid. Hence, if we suppose that this body is retained by 

 means of a thread, attached to the beam of a balance, and that 

 w is the weight with which it is kept in equilibrium ; we shall 

 have 



ro =S6_ S'b, 

 from which we obtain, 



Now Sb is the absolute weight of the body, and w its weight in 

 the fluid ; knowing, therefore, the absolute weight of a body, and its 

 weight when immersed in a fluid, we easily determine the ratio of their 

 specific gravities, by dividing by the absolute weight of the body the 

 difference of these two weights If, for example, the absolute 

 weight of a body be 6 ounces, and its weight when immersed 

 in water 5 ounces, we divide the absolute weight 6 by the diffe- 

 rence, and the quotient f shows that the specific gravity of the 

 body is 6 or is to that of the fluid as 6 is to 1 . 



In other words, since the loss of weight sustained by a body 

 on being immersed in a fluid is equal to the weight of its own bulk 

 of that fluid, by proceeding as above, we shall have the weight 

 of equal bulks of the substances in question, and consequently 

 their relative weights or specific gravities. 



If the solid substance whose specific gravity is sought be light- 

 er than water with which it is to be compared, it may be made 

 to sink by attaching to it a heavier body, whose water-weight is 

 known ; then, by subtracting this from the water weight of the 

 compound body, we shall have that of the body in question. 



When the substance to be weighed is soluble in water, as com- 

 mon salt, for instance, it may be covered with a thin coat of 

 melted wax, which is very nearly, and may be made exactly of 

 the same specific gravity with water. The body will thus be 

 protected, and the loss of weight, on being immersed, will be the 

 same as if water occupied the place of the covering. 



