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inches of air in the vessel at atmospheric pressure of 1 5 lbs. 
on the square inch ; and generally, n times 573 cubic inches 
of air forced into the cylinder would be the equivalent of 
n atmospheres of absolute pressure. 
In converse manner 5 lbs. of pressure or one third of an 
atmosphere is the equivalent of one third of 573 cubic 
inches, or the equivalent of 191 cubic inches of air at atmo- 
spheric pressure, and generally, 5 lbs. of pressure is the 
equivalent of 191 cubic inches of air at atmospheric pres- 
sure and for all the higher pressures. The mode of experi- 
menting was as follows : — Air was forced into the cylinder 
to the required density, and after the heat of compression 
had subsided, the time of each 51bs. reduction of pressure 
was taken by means of a half seconds pendulum, com- 
mencing its oscillations at the moment of discharge, and the 
stopcock was suddenly closed and the number of oscillations 
noted for every definite discharge and reduction of 5 lbs. of 
pressure. In my earlier experiments it was found that 
when the air was compressed to nine atmospheres and suc- 
cessive reductions of 5 lbs. were made to the lowest pressure, 
the cooling of the air produced a notable effect in diminish- 
ing the rate of discharge. By commencing the experiments 
with the lower pressures and increasing them by 10 lbs. 
successively after each discharge of 5 lbs., the changes of 
temperature attending the changes of density of the air 
were kept within the limits of 5 lbs. of pressure till the 
highest density was attained. The small changes of pressure 
attending each discharge by the addition and abstraction of 
heat to and from the cylinder were, after a little practice, 
easily corrected so that each discharge may well be con- 
sidered as having been made under conditions of constant 
temperature. The large cylinder B was first used as a 
vacuum chamber to receive the discharge from the small 
cylinder. The chamber was fitted with an exhausting 
pump and suitable vacuum gauges and the pressure within 
the chamber was reduced to six tenths of an inch of mer- 
