Chap. 4.] tiydroftatic Balance. Tf 



fubftances that may be eafily reduced to the fame bulk 

 or form may eafily be weighed and compared. By 

 weighing accurately a determinate quantity of any fluid, 

 an ounce for inftance, in a phial, and marking precifely 

 the fpace which it occupies in the phial, the weight of 

 the fame quantity of any other fluid may eafily be had 

 and compared with the former. 



The fpecific gravity of bodies which are not, nor 

 can eafily be reduced to an equal bulk, is not to be 

 obtained by any method equally obvious to imphilofo- 

 phical perfons. A method ,- however, has been invent- 

 ed for determining the fpecific gravities of folid bodies, 

 whatever their figure or dimenfions. As it is an ob- 

 vious principle, that every body when immerfed in a 

 fluid muft difplace a quantity of the fluid equal to its 

 own bulk, and the refiftance which it meets with from 

 the fluid will be found exactly equivalent to the weight 

 of the fluid fo difplaced ; hence if any fluid, as water 

 for inftance, is taken as the ftandard of connparifon, it 

 will be eafy to determine the fpecific gravity of dif- 

 ferent folids by weighing them firft accurately in air, 

 and afterwards weighing them in water, and comparing 

 their lofs of weight in this latter fluid, which will be 

 in exact proportion to the fpace which they occupy. 

 To make this clear by an experiment; fuppofe it was 

 neceffaryto determine the fpecific gravities of any two 

 metals, lead r-nd tin for inftance, I take a certain quan- 

 tity of the former, and weighing it carefully in air, I 

 find its weight amounts to thirty-four ounces; on 

 weighing it again in water, 1 find it weighs but thirty- 

 one ounces, that is, it has loft three ounces of its 

 weight, or in other words, the fame bulk of water 

 would weigh three ounces ; the fpecific gravity of lead 

 is therefore to that of water as 34 to 3 or as 1 1 -i to i. 

 On weighing a certain quantity of tin, I find again that 

 t it 



