-Sir oeorge shuckburgh’s Obfervathm 
■fchefe ideas I made the following experiment. I caufed a 
glais veflel to be blown fomething like a Florence flafk, 
or rather larger; to the neck of this was adapted a brafs 
cap with a valve opening outwards, and made to fcrew 
en or off, together with a male fcrew, by which it was 
fixed to an excellent pump of Mr. nairne’s conftrudlion, 
and exhaufted of its air, or at lead rarified to a known 
degree: the veflel was then carefully weighed with a 
fenfible balance, and again after the air was re-admitted; 
the difference gave the weight of the air that had been 
exhaufted. After having repeated this two or three 
times, the veflel was exadily filled with water as far as 
the valve, which had been the term of capacity for the 
air ; this was done by fcrewing on the cap till the fuper- 
fiuous water oozed all out, and upon inverting the veflel 
there appeared not the leaft fign or bubble of air; I 
therefore concluded, that the volume of water was pre- 
cifely. the fame as had been the volume of air, a circum- 
ftance that fhould be accurately attended to. It was then 
carefully weighed, and compared with its weight when 
full and deprived of its air. It will readily be feen, that 
I had then the fpecific gravity of the two fluids, upon 
fuppofition that the figure of the glafs had not altered 
Mr. boyle and Dr. halley, who both made ufe of this method ; the one with 
a -view to determine the limits of the atmofphere; and the other the height of 
Snowden. 
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