422 Vbe common Hydrometer. [Book VII. 



to be a fpecific gravity, about or nearly equal with 

 that of the fluid. 



If, therefore, the weight of the hydrometer is fuch, 

 that it will fink in water as far as E, it will fink ftill 

 deeper in lighter fluids ; it will fink for inftance in wine 

 as far as F ; in fpirit of wine as far as G, &c. But if 

 it is plunged into liquors heavier than water, it will 

 not fink fo far as Ej for example, in beer it will only 

 fink to D, and always as much lefs as the liquor into 

 which it is plunged is denfer and confequently heavier. 



By this procefs it will be eafily difcovered, whether 

 one kind of fluid is lighter than another with which it 

 is compared ; but the proportion cannot be accurately 

 determined i for in order to that, it would be neceflary 

 to know exactly the proportion between the tube A C 

 and the balls B and S ; which is impoflible according 

 to the above mode of conftr lifting the inftrument ; 

 and it would alfo be further neceflary that the tube AC 

 fhould be perfectly cylindrical, which never is the cafe. 

 The moft certain mode of acquiring this exact know- 

 ledge is, to operate always with equal volumes of 

 fluids : and for this purpofe, it is neceiTary to make ule 

 of Fahrenheit's hydrometer. 



This inftrument (fig. 2.) is compofed of a fmall 

 oval glafs bulb or bottle B -, the neck of which A C 

 is very fmall, and terminates with a baibn D E, de- 

 figned to receive fmall weights. 



The inftrument is ballafted by means of a fmall 

 ball of glafs S fixed underneath it, in which there is 

 depofited fome quickfilver. A fmall grain of fmalt(^) 

 is fixed upon the neck, and the inftrument is then 

 complete. 



In order to make ufe of this hydrometer, i.t is ne- 

 ceflary to begin by knowing its exact weight, which 



fliould 



