with which Air rushes into a Vacuum. 541 



would permit. We have therefore in the table a measure of 

 the difference of the theoretic and experimental velocities 

 with which air rushes into a vacuum by the same method of 

 calculation. This difference, as will be seen, amounts to 

 exactly one atmosphere of pressure. 



For each reduction of 5 lb. from 120 lb. to 40 lb. the times 

 of discharge are inversely as the pressures ; and as the density 

 of the issuing stream of air diminishes in the same proportion, 

 the velocity of discharge is the same for all the pressures from 

 120 lb. to 40 lb., as shown in the table. Hence it appeared 

 to me at the commencement of this investigation, that the 

 theoretic and experimental velocities with which air rushes 

 into a vacuum were rigorously exact. The anomalous and 

 apparent increase in the velocities from 40 lb. to 10 lb., how- 

 ever, led me to suspect that the atmosphere in some manner 

 affected the results, and induced me to make the discharge 

 into a vacuum with the results shown in Table I. 



That the phenomenal rate of discharge which I have de- 

 scribed should not hitherto have manifested itself in some 

 form, or be associated with some facts explanatory of it, 

 would indeed be surprising considering the varied circum- 

 stances in which the discharge of elastic fluids comes into 

 play. Hence, it has long been known that a jet of air issuing 

 from an aperture in a vessel produces a rarefaction of the 

 atmosphere near to the discharging orifice. This phenomenon 

 was first observed on a large scale by Mr. Richard Roberts in 

 the year 1824, and is described in a paper read before this 

 Society in 1828*. Roberts noticed that when a valve was 

 placed over an aperture in a pipe used for regulating a strong- 

 blast of air for blowing a furnace, the valve, instead of being- 

 blown off by the force of the blast, remained a short distance 

 from the aperture, and required considerable force of the hand 

 to remove it to a further distance. Subsequent experiments 

 showed that the adhesion of the valve was caused by the 

 partial vacuum formed between the valve and its seating by 

 the expansion of the issuing air. These experiments were 

 repeated and extended by Mr. Peter Ewart to similar effects 

 produced by the discharge of steam through various apertures. 

 Some of these experiments were described before this Society, 

 and afterwards published in the Philosophical Magazine in 

 18291*. The degree of rarefaction produced by the discharge 

 of air and high-pressure steam was carefully measured by 



* Memoirs of the Literary and Philosophical Society, 2nd series, vol. v. 

 p. 208. 



t u Experiments and Observations on some of the Phenomena attend- 

 ing the Sudden Expansion of Compressed Elastic Fluids." 



