NICHOLSON'S HYDROMETER. 79 



Con. If the second liquid be the standard, s' = 1, and 

 s, the specific gravity of the first liquid, is given in (1). 



39. Nicholson's Hydrometer. The two hydrometers 

 just described are used for obtaining the specific gravities of 

 liquids. Nicholson's hydrometer is so contrived as 

 to determine the specific gravity of solids as well 

 as liquids. 



It consists of a hollow metallic vessel C, gener- 

 ally of brass, terminated above by a very thin 

 stem, which is often a steel wire, bearing a small 

 dish A, and carrying at its lower end a heavy cup 

 D; on the stem connecting A and C, a well- 

 defined mark B is made. 



Fig. 29 



(1) To determine the specific gravity of a 

 liquid. 



Let w be the weight of the hydrometer, w' the weight 

 which must be placed in the dish A, in order to sink the 

 stem to the point B in a liquid whose specific gravity is s, 

 and w" the weight which must be placed in the dish A, to 

 sink the stem to the same point B in a liquid whose specific 

 gravity is s'. Then we have for the weights of the liquid 

 displaced in the first and second cases, respectively, 



w + w' and w -f- w" ; 



and since the volumes displaced are the same in both cases, 

 the specific gravities are as the weights (Art. 29), 



s^ _ w + w' 

 7' " w + w"' 



Calling the second liquid the standard, s' = 1, and (1) 

 becomes 



w -f w' 



S = - 77, (2) 



w + w 

 which is the specific gravity required. 



