120 HYDROSTATICS. 



under water. First weigh the body in air; then 

 having balanced the tongs in water, fix to it the 

 body to be weighed, which being lighter than water, 

 will raise the tongs, and cause the other scale to 

 preponderate. Observe the loss of weight of the 

 body in water, and proceed as before. 



There are some things that cannot be weighed 

 in this manner, such as quicksilver, fragments of 

 diamonds, &c. : these must be put into a glass 

 bucket hanging to the scale. 



The Hydrometer is the most eligible instrument 

 for finding the specific gravity of fluids. 



The most common hydrometer consists of a 

 copper ball, B, (Plate 8. Fig. 9.) to which is soldered 

 a brass wire, A B, one quarter of an inch thick. 

 The upper part of this wire being riled flat, is 

 marked proof, at m (Fig. 10.), because it sinks 

 exactly to that mark in proof spirits. There are 

 two other marks at A and B (Fig. 9.)» t0 show 

 whether the liquor be one-tenth above, or below 

 proof, according as the hydrometer sinks to A, or 

 rises to B, when a brass weight, C, is screwed to its 

 bottom. There are other weights to screw on, 

 which show the specific gravity of different fluids. 



The round part of the wire above the ball may 

 be marked so as to represent river-water when it 

 sinks to R W (Fig. 10.), the weight which answers 

 to that water being then screwed on j and when 

 put into spring- water, mineral- water, sea- water, and 

 water of salt springs, it will gradually rise to the 

 marks S P, M I, S E, S A; on the contrary, when 

 it is put into Bristol-water, rain-water, Port-wine, 

 and Mountain-wine, it will successively sink to the 

 marks b r, r a, p o, m o. 



Another sort of hydrometer is represented in 

 Plate 8. fig. 11.; which is calculated to ascertain 



