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XII. On the Refraction of Sound by the Atmosphere. 
Ry Professor Osborne Reynolds. Communicated by Professor Stokes, Sec. R.S. 
Received November 22, 1875, — Read January 6, 1876. 
In a paper read before the Royal Society, May 1874, I pointed out that the upward 
diminution of temperature iu the atmosphere (known to exist under certain circum- 
stances) must refract and give an upward direction to the rays of sound which would 
otherwise proceed horizontally; and it was suggested that this might be the cause 
of the observed difference in the distinctness with which similar sounds are heard on 
different occasions, particularly the very marked advantage which night has over day 
in this respect. At the time at which that paper was written no direct experiments or 
observations had been made to verify the truth of this suggestion, and therefore its 
probability rested on its reasonableness. Since that time, however, I have carried out 
a series of observations and experiments which, although far from complete, throw some 
light on the subject, besides revealing some remarkable facts. I hope to be able to 
continue the investigation ; but since its nature is such as to render the chance of bringing 
it to any thing like a final conclusion very uncertain, it seems to me that it may be 
well to publish an account of what has been already done ; and this is the object of the 
present communication. 
In order to render the object of the various experiments clear, it may be well to 
recapitulate here some of the theoretical considerations previously explained. It will be 
remembered that the idea that the variations of temperature would cause refraction of 
sound occurred to me while making experiments on the effect of wind upon sound, from 
which it was shown that when sound proceeds in a direction contrary to that of the 
wind, it is not, as had been thought, destroyed or stopped by the wind, but that it is 
lifted, and that at sufficiently high elevations it can be heard to as great distances as in 
other directions, or as when there is no wind — thus confirming the hypothesis first pro- 
pounded by Professor Stokes and afterwards by myself, that the effect is owing to the 
retardation of the velocity of the wind near the earth, which allows the sound moving 
against the wind to move faster below than above, and thus causes the fronts of the 
waves to incline upwards, and consequently to move in that direction. Having clearly 
shown that this was the case, it became apparent that any thing which would cause an 
upward diminution in the velocity at which sound proceeds would cause a similar effect 
to that of the wind and lift the sound, and that since the speed of the sound depends 
on the temperature of the air in which it is moving, an upward diminution in the tem- 
perature must cause such an effect. That such a diminution of temperature does very 
