Chap, a.]' *The Hydrometer. 421 



cording to the denfity of the water. Hence, as water 

 when impregnated with fait, is heavier and denfer than 

 pure water, as was formerly proved * ; it follows, that 

 the fame boat and lading will .draw lefs water (ac- 

 cording to the feaman's phrafe) in the ocean, than in 

 a river of frefh water, and that if: it is laden to the utmoft 

 extent in fait water, it will fink when it comes into frem 

 water. 



Upon this principle depends the life of the hydro- 

 meter. For it will be found that bodies immerfed in 

 mercury, fuftam a greater lofs of weight, or require 

 a greater power to fink them, than in fait water ; in 

 fait water than in frefh ; and in pure water, they ftill 

 fuftain a greater lofs of weight than in fpirit of wine, 

 which is a lighter and lefs denle fluid. 



The hydrometer, then, is an inftrurnent by which 

 the fpecific gravity of different fluids is determined. 

 The moft fimple, and that which is moft in ufe, con- 

 fifts of a glafs globe with a long narrow neck (plate 

 VIL fig. i.) AC, divided all the way up into 

 equal parts. In order that this inftrument may fuf- 

 tain icfelf in the middle of liquors in a vertical pofi- 

 tion, it is made in fuch a manner that the center of 

 gravity is in the lower part of it j it is for this purpofe, 

 that another fmall hollow ball S is placed under the 

 larger ball, in which lome mercury is ufually lodged, 

 but in fuch a quantity only as that the whole hydro- 

 'meter mall weigh about as much as an equal bulk of 

 tha-t fluid which is appointed to be proved. 



When the hydrometer is conftructed in this manner, 

 if it is plunged in the liquors which are to be com- 

 pared, it will not entirely fink, becaufe it is fuppofcd 



* Book vi. chap. 7. 



Ee 3 to 



