for meajuring the Specific Gravity of Bodies, 387 



mention previoufly what has been done in this 

 way. 



It feems to follow from a paflage in Boyle's 

 account of a new Effay Inftrumenc, * that the 

 Hydrometer, or Areometer was firfl invented by 

 that great philofopher. The effay inftrument 

 here mentioned, was intended for the hydrofta- 

 tical proof of metals, and was adapted to ferve 

 chiefly for guineas. It confided of a ball, fome- 

 what lefs than an hen's egg, with a ftem of four 

 or five inches in length, foldered to the upper 

 part, and a bent wire or ftirrup beneath, to place 

 the coin upon. A flit piece of brafs, with a 

 lateral fcrew to hold the coin tight, though in 

 fa6t conducing more to the eafe than accuracy of 

 the experiment, is mentioned by the author, as 

 being preferable to the ftirrup : and, to extend 

 the ufe of the inftrument, he propofes that the 

 ball be made large, and provided with a contri- 

 vance for occafionally changing the quantity of 

 ballaft applied beneath the ball. 



Boyle's inftrument was intended to be ufed in 

 water, and, confequently, the graduations of its 

 ftem denoted certain invariable weights. But 

 when the hydrometer is to be ufed in various 

 fluids, it diminifties the accuracy of the refults, 



• Lowthorp's Abridgement of the Philofophical Tranfac- 

 tions, vol. I. p. 604. Or Boyle's Works in /^.to. edit, 

 London, 1772, vol. IV. p. 204. 



C c 2 if 



