27 
On comparing the times of discharge in Table III. and 
the velocities calculated therefrom with the times and 
velocities in Table II. a remarkable difference will be 
observed in them for the same effective pressures. Thus 
the velocity of discharge from 15lbs. to 141bs. appears to be 
double that assigned to the same pressure when the dis- 
charge is made into a vacuum : while in the discharge from 
21bs. to lib. (the lowest pressure in the table) the velocity 
appears to be more than six times greater, or 4219 feefc per 
second. No less remarkable than this apparent increase in 
the rate of discharge is the complete inversion of the order 
of the velocities as compared with those when the discharge 
was made into a vacuum for the same effective pressure. 
Now, we have knowledge of several causes competent to 
diminish the velocity of air of constant temperature flowing 
into the atmosphere, but none to increase the velocity 
except the form of the aperture, which in this case remained 
unchanged. Recognising the fact that when air of l-51bs. 
effective pressure was discharged into the atmosphere, the 
cylinder actually contained two atmospheres of absolute 
pressure, we are led to the conclusion that the phenomenal 
increase in the rate of discharge observed is caused by the 
external atmosphere acting as a vacuum, and offers no 
resistance to the discharge into it of air of 15lbs. pressure, 
which thereby becomes 301bs. effective pressure. The 
velocity of air of lolbs. effective pressure discharged into 
the atmosphere based on this conclusion is 1021 feet per 
second, the same as the velocity found for the discharge 
into a vacuum. For effective pressures below 151bs, the 
velocities are compounded of the rate of discharge into a 
vacuum, and the resistance of the atmosphere without any 
regular ratio, but approximating to the square roots of the 
pressures. ' 
That the atmosphere acts as a vacuum to the discharge of 
air into it of 151bs. effective pressure is further evident from 
the results obtained and shown in Table IV. 
