On the Velocity with which Air rushes into a Vacuum. 531 



and 5000 millim., and the numbers found vary only between 

 1*089 and 1*091. The data for ethyl iodide are much less 

 complete; the comparison with ethyl chloride can only be 

 made at pressures between 150 and 500 millim., and with 

 ethyl bromide between 50 and 500 millim. In the first case 

 the ratio of the absolute temperatures varies between 1*206 

 and 1*209, and in the second case between 1*105 and 1*108. 

 [To be continued.] 



LXII. On the Velocity with which Air rushes into a Vacuum, 

 and on some Phenomena attending the Discharge of Atmo- 

 spheres of Higher into Atmospheres of Lower Density. By 

 Henry Wilde, Esq.* 



GONSlDEEINGr the present condition of our knowledge 

 respecting the mechanical properties of air and other 

 gases, some apology might appear to be needed in bringing 

 before this Society the results of an investigation touching 

 some fundamental principles in pneumatics, which for more 

 than a century have been considered to rest on foundations 

 as secure as the laws of gravitation of the heavenly bodies. 

 A survey of the history of the dynamics of elastic fluids will, 

 however, show that, great as are the advances w T hich have 

 been made in this branch of science, the laws of the discharge 

 of elastic fluids under the varied conditions of elasticity and 

 volume are still left in much obscurity. The several circum- 

 stances which have combined to produce this anomalous state 

 of our knowledge of this subject are : — (1) The application of 

 the laws of discharge of inelastic fluids, without any modifica- 

 tion, to those which are elastic ; (2) the confusion of the 

 • quantity of the discharge of elastic fluids after leaving the 

 vessel, with the velocity of discharge through the aperture in 

 the vessel ; and (3) the want of a sufficient number of expe- 

 riments, under varied conditions and through sufficient range of 

 pressure, to compare with the deductions derived from theory. 



It has hitherto been assumed, as a leading proposition in 

 pneumatics, that air rushes into a vacuum with the velocity 

 which a heavy body would acquire by falling from the top of 

 a homogeneous atmosphere of the same density as that on the 

 earth's surface ; and since air is about 840 times lighter than 

 water, if the whole pressure of the atmosphere be taken as 

 equal to support 33 feet of water, we have the height of the 

 homogeneous atmosphere equal to 27,720 feet, through which, 

 by the free action of gravity, is generated a velocity of 1332 



* Communicated by the Author, having been read at a Meeting of the 

 Manchester Literary and Philosophical Society, October 20, 1885. 



