with ivhich Air rushes into a Vacuum. 535 



of air forced into the cylinder would be the equivalent of n 

 atmospheres of absolute pressure. 



In converse manner, 5 lb. 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 atmospheric 

 pressure ; and, generally, 5 lb. of pressure is the equivalent 

 of 191 cubic inches of air at atmospheric pressure and for all 

 the higher pressures. The mode of experiment was as fol- 

 lows : — Air was forced into the cylinder to the required density, 

 and after the heat of compression had subsided, the time of 

 each 5 lb. reduction of pressure was taken by means of a half- 

 seconds pendulum, commencing 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 lb. of pressure. In my earlier experiments, it 

 was found that when the air was compressed to nine atmo- 

 spheres, and successive reductions of 5 lb. were made to the 

 lowest pressure, the cooling of the air produced a notable 

 effect in diminishing the rate of discharge. By commencing 

 the experiments with the lower pressures and increasing them 

 by 10 lb. successively after each discharge of 5 lb., the changes 

 of temperature atttending the changes of density of the air 

 were kept within the limits of 5 lb. of pressure till the highest 

 density was attained. The small changes of pressure attend- 

 ing each discharge by the addition and abstraction of heat to 

 and from the cylinder were after a little practice easily cor- 

 rected, so that each discharge may well be considered 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 mercury ; and that degree of vacuum 

 was maintained during the experiments. 



The following table shows the velocity of air flowing into a 

 vacuum, as deduced from the time and difference of pressure 

 for every 5 lb. from 135 lb. to 5 lb. absolute pressure. The 

 velocities of the first column are deduced from actual experi- 

 ment, and in the next column the velocities are calculated 

 from the difference of the area of the discharging orifice and 

 the vena contracta by applying the hydraulic coefficient '62. 



2P2 



