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ON THE REFRACTION OE SOUND BY THE ATMOSPHERE. 
I have met with instances at night which have led me to apply the same explanation. 
Many of the phenomena, however, to tvhich Dr. Henry has applied this explanation 
are, I feel sure, to be attributed to the effect of the upward variation of temperature.. 
Dr. Henry does not appear to have been aware of this cause of refraction of sound 
while making his experiments or drawing up his Report ; but in a note at the end he 
expresses his general agreement with the views stated in my previous paper. 
The Heterogeneity of the Atmosphere. 
With respect to the stoppage of the sound by the heterogeneity of the atmosphere, 
Dr. Henry expressly states that through all his long experience he has never met with 
a single phenomenon which he can fairly ascribe to this cause ; and so far as my ex- 
perience goes it agrees with that of Dr. Henry. I am far, however, from thinking that 
there is no such effect ; on the contrary, under circumstances such as those which 
Humboldt describes as having led him to the idea, it seems to me that it must exist, but 
that it must at all times be confined to a very small distance above the earth’s surface 
and be over land. That it is the principal cause, or even an important cause of the 
phenomena under discussion, appears to be more than doubtful ; for not only does the 
necessary effect of refraction appear to be a sufficient cause for these phenomena, and 
therefore to afford a complete explanation of them, but it is very difficult to conceive 
the existence of a state of heterogeneity in a calm clear atmosphere at a considerable 
elevation above the level of the sea. 
In the first place such a state of heterogeneity could hardly fail to be observed ; for it 
would necessarily impart a flickering and unsteady appearance to objects seen through 
it — an effect which may be observed any hot summer’s day when looking at objects low 
down over dry land. Over the sea, however, such an appearance has not been recorded; 
and although I have often looked for it, I have been entirely unable to detect it. And 
in the second place, even supposing the air to be in a heterogeneous state at any given 
instant, such a state could not be maintained many minutes ; for different gases, or 
different portions of the same gas at different temperatures, mix and diffuse very rapidly. 
It is true that the heterogeneity might be maintained by upward streams of heated air 
or vapour, and this is doubtless the cause of the heterogeneity of air over dry hot 
ground ; but this heterogeneity, although very apparent near the ground, is never 
observed at any considerable height. Upward streams of heated ah must tend to mix 
and diffuse rapidly, and the ah as it rises is cooled by expansion until it must soon 
cease to be lighter than the surrounding air. That, as a rule, there are no streams of 
heated air ascending to any considerable height over land, is definitely proved by the 
fact that the light smoke from burning weeds never, or very seldom, attains an eleva- 
tion of any thing like 100 feet. I have often been struck with the way in which such 
smoke will creep along the ground for the distance of half a mile, and even then not 
extend to an elevation of more than 20 or 50 feet. Over the sea the cause of such 
streamlets must be much less potent than over land, and their existence still more 
unlikely. 
