HYDROSTATICS. 121 



the specific gravity of fluids with greater pre- 

 cision; it consists of a large hollow ball, B, with a 

 smaller bolt, b, screwed on to its bottom, partly filled 

 with mercury, or small shot, in order to render it 

 but little specifically lighter than water. The 

 larger ball has also a short neck at C, into which is 

 screwed the graduated brass wire, A C, which, by a 

 small weight at A, causes the body of the instru- 

 ment to descend in the fluid with part of the stem. 

 When this instrument is swimming in the liquor 

 contained in the jar, I L M K, the part of the fluid 

 displaced by it will be equal in bulk to the part of 

 the instrument under water, and equal in weight 

 to the whole instrument. Now, suppose the weight 

 of the whole to be 4000 grains, it is evident we can 

 by this means compare the different dimensions of 

 •1000 grains of several sorts of fluids. For if the 

 weight at A be such as will cause the ball to sink 

 in rain-water, until its surface come to the middle 

 point of the stem, 20; and after that, if it be im- 

 mersed in common spring- water, and the surface be 

 observed to stand at one-tenth of an inch below 

 the middle point, 20, it is apparent that the same 

 weight of each water differs only in bulk, by the 

 magnitude of one-tenth of an inch in the stem. 



Now, suppose the stem to be ten inches long, 

 and weigh 100 grains, then every tenth of an inch 

 will weigh one grain ; and as the stem is of brass, 

 which is about eight times heavier than water, the 

 same bulk of water will be equal to one-eigthth of 

 a grain, and consequently to one-eigthth of 75^ 

 part; that is, ttW part of the whole bulk. This 

 instrument is capable of still greater precision, by 

 making the stem or neck consist of a flat thin slip 

 of brass, instead of one that is cylindrical; for bv 

 this means we increase the surface, which is the 



