WITH WHICH AIR RUSHES INTO A VACUUM. 151 



I have said that the capacity of the cylinder A was 573 

 cubic inches^ which represents the same number of cubic 

 inches of air in the vessel at atmospheric pressure of 1 5 lb. 

 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 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 1 9 1 cubic inches of air at atmospheric 

 pressure and for all the higher pressures. The mode of 

 experiment 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 5 lb. reduction 

 of pressure was taken by means of a half-seconds pendu- 

 lum, commencing its oscillations at the moment of dis- 

 charge; 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 experi- 

 ments, it was found that when the air was compressed to 

 nine atmospheres, and successive reductions of 5 lb. were 

 made to the lowest pressure, the cooling of the air pro- 

 duced 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 dis- 

 charge of 5 lb., the changes of temperature attending 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 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 considered as having been 

 made under conditions of constant temperature. The 



