38 
by the sliding of the layers. If a plane layer is given in 
the state of minimum density, there are 3 sets of plane 
layers in minimum and 8 in maximum density. If a plane 
layer is given in the state of maximum density, it does not 
follow that there are any such directional properties in the 
mass. — R F. G. 
“Note on the Velocity with which Air rushes into a 
Vacuum, and on some Phenomena attending the Discharge of 
Atmospheres of higlier, into Atmospheres of lower Density,” 
by Henry Wilde, Esq. 
Since the reading of my paper before the Society on the 
efflux of air, I have thought that it might be useful to 
recapitulate, briefly, the fundamental grounds upon which 
my experiments and the general reasoning thereon were 
based. This appears to me to be further necessary, from 
the dual sense in which the term “velocity” may be 
considered in the discharge of elastic fluids : — the term, as I 
have already pointed out, has been applied by some, in- 
differently, to express the rate of increase of volume after 
leaving the aperture, and the velocity of the stream through 
the aperture before expansion. It is in the latter sense 
that the term is used in my paper, and the velocities shown 
in the several tables have all been calculated on this basis. 
The application of the laws of discharge of inelastic fluids 
to those which are elastic, is a natural principle of reasoning 
sufflcient for us to assume a theoretic velocity for air rushing 
into a vacuum of 1332 feet per second; and the corollary 
to this proposition, that the velocity of efflux through the 
aperture into a vacuum is the same for all pressures above 
and below that of the atmosphere also follows, naturally 
and directly, from the reciprocal relations of the elasticity 
and density of the homogeneous atmosphere. But, just as 
the theoretic velocity of discharge of water and other 
inelastic fluids is diminished by the opposing motions and 
