542 Mr. H. Wilde on the Velocity 



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 1842*, 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 requisite to draw it out of 

 the jet. 



It is abundantly evident from these experiments, that when- 

 ever 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 equili- 

 brium with the external pressure, has a high degree of proba- 

 bility ; but that this vacuous space should have the effect of 

 increasing the rate of discharge could only be ascertained, as 

 we have seen, by a direct comparison, under like conditions, 

 with the amount of the discharge into a vacuum. 



Having established the fact that the atmosphere acts as a 

 vacuum to the discharge of air of all pressures above two 

 atmospheres within the range of my experiments, it appeared 

 to me that this phenomenon might only be a particular case 

 of a general law of the discharge of elastic fluids, and that it 

 would be interesting to know through what range of relative 

 pressures in two vessels the one would act as a vacuum to the 

 other. With this object air was compressed into the large 

 receiving cylinder from two up to eight atmospheres absolute 

 pressure, while air was condensed into a small discharging 

 cylinder up to nine atmospheres of absolute pressure. The 

 air was discharged from the same orifice as in the former 

 experiments, and the time of discharge recorded for each 

 atmosphere was for a reduction of 5 lb. of pressure. The 

 results obtained are shown in the table. 



* a On the Efficacy 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. 1. 



