OF THE EQUILIBRIUM OF FLOATATION. 281 



therefore, according to the second clause of the rule, the part immersed 

 when the body floats in vacuo, becomes 



0.925(1 0.0012) 

 = X171 - 108: 



= 



353. If we refer to the equation (218) preceding, it will readily 

 appear, that the above result may be determined with much less labour 

 and greater simplicity ; for the magnitude of the immersed part, when 

 the body floats in vacuo, is there expressed in terms of the weight of 

 the body and the specific gravity of water, and the practical rule for 

 reducing the equation, may be expressed in words at length, as follows. 



RULE. Multiply the magnitude of the body by its specific 

 gravity ; then divide the product by the specific gravity of 

 water, and the quotient will express the magnitude of the 

 immersed part when the body floats in vacuo. 



Therefore, by taking the data as proposed in the above example, 

 the magnitude of the immersed part becomes 



185X0.925 

 m'~ - - - ^ = 171.125 cubic inches; being precisely 



the same quantity as we obtained by the foregoing prolix operation. 



If we compare the computed values of m' and m" with one another, 

 we- shall find that the latter exceeds the former by a very small quan- 

 tity, that is, 



171.125 171. 108 =z 0.017, 



which verifies the concluding inference under Problem L. 



354. On the principles which we have explained and illustrated in the 

 foregoing problems, depends the construction and application of the 

 Hydrometer, an instrument which is generally employed for detect- 

 ing and measuring the properties and effects of water and other fluids, 

 such as their density, gravity, force, and velocity. 



When the hydrometer is employed to determine the specific gravity 

 of water, it is sometimes denominated an aerometer or water-poise ; 

 and being an instrument of very general utility in numerous philoso- 

 phical experiments, we think it will not be amiss in this place, to 

 discuss its nature and properties a little in detail ; and we may here 

 observe, that the following problems and remarks are quite sufficient 

 to establish and exemplify its most important applications. 



The hydrometer, or aerometer, in general consists of a long cylin- 

 drical stem of glass, or other metal, connected with two hollow balls, 



