OF THE SPECIFfC GRAVITIES OF BODIES. 189 



From this we can easily find the weight of a ship, or LECT. 

 any other body that floats in water. For, if we mul- *^^^s 



How the 

 weight of 

 fluid displaced by it, will be equal in bulk to the part of the instru- a s hip I Q 



ment under water, and equal in weight to the whole instrument Now, water may 

 suppose the weight of the whole to be 4000 grains, it is evident we " e estuna- 

 can by this means compare the different dimensions of 4000 grains of 

 several sorts of fluids. For if the weight on the top be such as will 

 cause the ball to sink in rain-water, until its surface come to the mid- 

 dle point of the stem ; and after that, if it be immersed in common 

 spring-water, and the surface be observed to stand at one-tenth of an 

 inch below the middle point, 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, if we 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-eighth of a grain, and conse. 

 quently to one-eighth of ^^th part ; that is, H&jgth 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, by this means we increase the surface, which is 

 the most requisite circumstance, and diminish the solidity, which ne- 

 c essarily renders the instrument still more accurate. 



To adapt this instrument to all purposes, there should be two stems 

 to screw on and off. One stem should be a smooth thin slip of brass, 

 or rather steel, like a watch-spring set straight ; on one side of which 

 are to be the several marks or divisions, to which it will sink in dif- 

 ferent sorts of water ; as rain, river, spring, sea, and salt spring 

 waters, &c. and on the other side you may mark the divisions to which 

 it sinks in various lighter fluids ; as hot Bath-water, Bristol-water, 

 Lincomb-water, Cheltenham-water, Port-wine, Mountain, Madeira, 

 and other sorts of wines. But here the weight on the top must be a 

 little less than before, when it was used for heavier waters. 



In trying the strength of the spirituous liquors, a common cylindri- 

 cal stem will do best, because of its strength and steadiness : and this 

 ought to be so contrived, that, when immersed in what is called proof- 

 spirit, the surface of the spirit may be upon the middle point; 

 which is easily done by duly adjusting a small weight on the top, and 

 making the stem of such a length, that, when immersed in water, it 

 may just cover the ball, but when immersed in pure spirit, it may rise 

 to the top. Then, by dividing the upper and lower parts into ten 

 equal parts each, when the instrument is immersed in any sort of 

 spirituous liquor, it will immediately shew how much it is above or 

 below proof. 



