ai 
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 increasing the rate of dis- 
charge 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 
the 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 dis- 
charge recorded for each atmosphere was for a reduction of 
51bs. of pressure. The results obtained are shown in the 
Table. 
