202 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



weight of the body in air. The solid body is then 

 transferred to the lower cone of the instrument, whose 

 upper surface is flat for this purpose. The water is 

 no longer at the mark on the stem, since the instru- 

 ment is lighter by an amount represented by the 

 water displaced by the body. The weights put 

 on to bring the hydrometer to the level of the mark 

 give the weight of the displaced water whose 

 volume is equal to that of the solid body. The 

 weight of the body in air divided by the weight 

 of the displaced water is the specific gravity of 

 the body. For measuring the specific gravity of 

 liquids Nicholson's hydrometer may be used in a way 

 similar to the hydrostatic balance. Thus the hydro- 

 meter is immersed in water and loaded till brought 

 down to the mark on the stem. The weight of the 

 instrument and the weights which it carries in the 

 pan are equal to the weight of the volume of water 

 it displaces. Immerse it now in the liquid to be 

 examined, and load it again till it is down in the 

 liquid to the proper level. Again the weight of the 

 instrument and the weights in the pan a,re equal to the 

 weight of the volume of liquid it displaced. In both 

 cases the volumes are the same ; therefore, the latter 

 result divided by the former gives the specific gravity. 

 In other words, the weight of the instrument being in 

 both cases the same, the amount of weight in the pan 

 011 the second trial, divided by the amount on the 

 first, is the required specific gravity. The estimation 

 of the specific gravity of liquids in this way is 

 better performed by the HYDROMETER OF FAHRENHEIT, 

 which is made of glass so as not to be attacked by 

 the liquids in which it is immersed. It is of similar 

 shape to Nicholson's, the hollow cylinder being formed 

 of glass blown out to the proper shape and size, and 

 being continuous below with a small bulb containing 

 mercury, for maintaining the vertical position. No 



