OF FLUIDS ON VIBRATING ELASTIC SURFACES. 
339 
124. If this be so, then a plate vibrating in the atmosphere may have the 
air immediately in contact with it separated into numerous portions, forming- 
two alternating sets like the heaps described (95) ; the one denser, and the other 
rarer than the ordinary atmosphere ; these sets alternating with each other by 
their alternate expansion and condensation with each vibration of the plate. 
125. With the hope of discovering some effect of this kind, a flat circular 
tin plate had a raised edge of tin three quarters of an inch high fixed on all 
round, and the plate was then attached to a lath (69), a little lycopodium put 
on to it, and vibrated powerfully, so that the powder should form a mere cloud 
in the air, which, in consequence of the raised edge and the equal velocity (70) 
of all parts of the plate, had no tendency to collect. Immediately it was seen 
that in place of a uniform cloud it had a misty honeycomb appearance, the 
whole being in a quivering condition ; and on exerting the attention to perceive 
waves as it were travelling across the cloud in opposite directions, they could 
be most distinctly traced. This is exactly the appearance that would be pro- 
duced by a dusty atmosphere lying upon the surface of a plate and divided 
into a number of alternate portions rapidly expanding and contracting simul- 
taneously. 
126. But the spaces were very many times too small to represent the inter- 
val through which the air by its elasticity would vibrate laterally once for two 
vibrations of the plate, in analogy with the phenomena of liquids ; and this 
forms a strong objection to its being an effect of that kind. But it does not 
seem impossible that the air may have vibrated in subdivisions like a string or 
a long column of air; and the air itself also being laden with particles of lyco- 
podium would have its motions rendered more sluggish thereby. I have not 
had time to extend these experiments, but it is probable that a few, well chosen, 
would decide at once whether these appearances of the particles in the air are 
due to real lateral vibrations of the atmosphere, or merely to the direct action 
of the vibrating plate upon the particles. 
127- If the atmosphere vibrates laterally in the manner supposed, the effect 
is probably not limited to the immediate vicinity of the plate, but extends to 
some distance. The vertical plates intersecting the surface of water and vibra- 
ting in a horizontal plane (117) produced ripples proceeding directly out from 
them five or six inches long ; whilst the waves parallel to the vibrating plate 
