for me-ifuring the SpeJJic Gravity of Bodies. 389 



allow for the diminution of fpecific gravity, 

 which arifes from the thermometrical expanfion 

 of fluids, is ufed by the officers of excife. 



This hydrometer is inferior to Fahrenheit's 

 in two refpeds. In the firft place, either a 

 bubble of air, or a portion of the fluid, vixW lie 

 hid in that part of the cavity of the ballaft 

 weight, which is not filled by the fcrew; and ic 

 is of very different confequence, which of the 

 two is there. And fecondly, the weights afling 

 on the inftriiment, by their refidual gravity, 

 will not be conftant ; or, in other words, an 

 additional weight will be accompanied by an 

 addition to the bulk of the immerfed part of the 

 inftrument : and, in the cafe where the fpecific 

 gravity of the liquid is not given, but required, 

 it will not be eafy to determine how much the 

 operation of the one is counteraded by that of 

 the other. However, though this lart: confider- 

 ation evinces that the inflrrument is not fit for 

 general ufe, yet it is accurate for the trial of 

 ardent fpirit, or any other particular liquid, 

 when the weights are adjulled by experiment to 

 the intended ufe. 



Pofl:erior to thefe, there have been feveral at- 

 tempts to improve the hydrometer, but as thiy 

 have been aimed chiefly to render it more per- 

 fect or convenient, with refpetSt to the fingle ufe 

 of proving fpirits, it is unneceflTary to dcfcribe 

 them at large. Among thefe it is hov/ever 

 proper to mention thofe of Dr. G. t'ordyce, and 



C c 3 Mr. 



