160 MR. II. WILDE ON THE VELOCITY 



through various apertures. Some of these experiments 

 were described before this Society, and afterwards published 

 in the Philosophical Magazine in 1829^. The degree of 

 rarefaction produced by the discharge of air and high- 

 pressure steam was carefully measured by Ewart by means 

 of gauges inserted in different parts of the jet. He also 

 noticed the sudden fall of temperature from 292° to 189° 

 F. in the rarefied part of the jet when steam of 58 lb. 

 pressure was discharged into the atmosphere. 



Sir William Armstrong also, in his experiments on 

 Hydro-electricity in the year i842tj described a singular 

 effect of a jet of steam by which a hollow globe made of 

 thin brass, from two to three inches in diameter, remained 

 suspended in a jet of high-pressure steam issuing from an 

 orifice; and when the ball was pulled on one side by 

 means of a string, a very palpable force was found requi- 

 site to draw it out of the jet. 



It is abundantly evident from these experiments, that 

 whenever elastic fluids escape into the atmosphere a 

 partial vacuum is formed near to the discharging orifice, 

 the degree of vacuum depending on the density of the 

 issuing stream. Ewart^'s ingenious explanation, that the 

 vacuous space formed near the discharging orifice is caused 

 by the joint action of elasticity and momentum of the 

 suddenly released particles repelling each other beyond 

 the distance necessary to produce equilibrium with the 

 external pressure, has a high degree of probability ; but 

 that this vacuous space should have the effect of increas- 

 ing the rate of discharge could only be ascertained, as we 



* " Experiments and Observations on some of the Phenomena attending 

 the Sudden Expansion of Compressed Elastic Fhiids." 



t " On the EfSeacy of Steam as a Means of producing Electricity, and 

 on a Curious Action of a Jet of Steam upon a Ball," Phil. Mag. ser. 3, 

 vol. xxii. p. I. 



