448 
PEOEESSOE STOKES ON THE COMMUNICATION OE VIBEATION 
This explanation never satisfied me, believing, as I always have done, for reasons 
which it would take too long here to explain, that for purely hydrodynamical phenomena 
(such as those of sound) an intimate mixture of gases was equivalent to a single homo- 
geneous medium. I had some idea of repeating the experiment, thinking that possibly 
Leslie might not have allowed sufficient time for the gases to be perfectly mixed, 
though that did not appear likely, when another explanation occurred to me, which 
immediately struck me as being in all probability the true one. 
In reading some years ago an investigation of Mr. Eaknshaw’s, in which a certain 
result relating to the propagation of sound in a straight tube was expressed in terms 
among other things of the velocity of propagation, the idea occurred to me that the 
high velocity of propagation of sound in hydrogen would account for the result of 
Leslie’s experiment, though in a manner altogether different from anything relating to 
the propagation of sound in one dimension only. 
Suppose a person to move his hand to and fro through a small space. The motion 
which is occasioned in the air is almost exactly the same as it would have been if the 
air had been an incompressible fluid. There is a mere local reciprocating motion, in 
which the air immediately in front is pushed forwards, and that immediately behind 
impelled after the moving body, while in the anterior space generally the air recedes 
from the encroachment of the moving body, and in the posterior space generally flows 
in from all sides, to supply the vacuum which tends to be created ; so that in lateral 
directions the motion of the fluid is backwards, a portion of the excess of fluid in 
the front going to supply the deficiency behind. Now conceive the periodic time of 
the motion to be continually diminished. Gradually the alternation of movement 
becomes too rapid to permit of the full establishment of the merely local recipro- 
cating flow ; the air is sensibly compressed and rarefied, and a sensible sound-wave (or 
wave of the same nature, in case the periodic time be beyond the limits suitable to 
hearing) is propagated to a distance. The same takes place in any gas ; and the more 
rapid be the propagation of condensations and rarefactions in the gas, the more nearly 
will it approach, in relation to the motions we have under consideration, to the condition 
of an incompressible fluid ; the more nearly will the conditions of the displacement of 
the gas at the surface of the solid be satisfied by a merely local reciprocating flow. 
This explanation when once it suggested itself seemed so simple and obvious that I 
could not doubt that it afforded the true mode of accounting for the phenomenon. It 
remained only to test the accuracy of the assigned cause by actual numerical calculation 
in some case or cases sufficiently simple to permit of precise analytical determination. 
The result of calculations of the kind applied to a sphere proved that the assigned cause 
was abundantly sufficient to account for the observed result. I have not hitherto pub- 
lished these results ; but as the phenomenon has not to my knowledge been satisfac- 
torily explained by others, I venture to hope that the explanation I have to offer, sim- 
ple as it is in principle, may not be unworthy of the notice of the Royal Society. 
For the purpose of exact analytical investigation I have taken the two cases of a vibra- 
