320 
PROFESSOR OSBORNE REYNOLDS ON THE 
variation of temperature was the reverse of what I wanted ; and before leaving the yacht 
I had my doubts of our finding much upward refraction of sound — of our being able to 
hear the rockets further than the guns. I was in hopes, however, that as the sun came 
out matters might change, and while in the boat I kept looking out for signs of de- 
pression in the distant objects. These, however, never came ; they loomed all the time, 
and very considerably. From the boat we could see the water for five or six miles. The 
yacht’s hull was visible to us all the time. On one occasion we had two buoys and a 
ship in a line, the nearest buoy being two miles from us ; we could see the water between 
this and the second, and again between this and the ship. 
It seems to me, therefore, that although in a manner the reverse of what was ex- 
pected, our observations this day prove the very great effect which upward refraction 
has on the distances at which sounds can be heard. The looming of the distant objects 
showed that the air was colder below than above. This would tend to bring the sound 
down and intensify it at the surface of the water — in fact convert the sea into a 
whispering-gallery. 
No other explanation appears to hold good. The conditions were exactly those which 
have been described as favourable to acoustic opacity ; the sea was calm, there was no 
wind, and an August sun was shining with its full power, and, having evaporated the 
clouds, must have been raising vapour from the sea. 
During the experiment I particularly noticed the echoes. Except the first and only 
pistol, none of the reports were attended with echoes or reverberation. But in most 
cases, though not in all, after calling I could hear the ring of my voice for ten or 
eleven seconds; and on one or two occasions when there were boats within half a mile 
of us, I could distinctly hear the echoes from them. Without attempting to explain the 
reverberation and echoes which have been observed, I will merely call attention to 
the fact that in no case have I heard any attending the reports of the rockets, although 
they seem to have been invariable with the guns and pistols. This fact suggests that 
these echoes are in some way connected with the direction given to the sound. They 
are caused by the voice, trumpets, and the siren, all of which give direction to the 
sound ; but I am not aware that they have ever been observed in the case of a sound 
which has no direction of greatest intensity. 
Arago’s Experiments. 
These observations in Lynn Deeps were the last I made in 1874. In the spring of 
this year my attention was called to a phenomenon recorded by Arago, which was 
noticed during the celebrated experiments on the velocity of sound made by Humboldt, 
Arago, Proxy, Gay-Lussac, and others, on the nights of the 21st and 22nd of June, 
1822, between Villejuif and Montlhery. On both these nights the sounds from 
Montlhery were heard more distinctly at Villejuif than the sounds from Villejuif at 
Montlhery, although the wind was blowing (very lightly) from Villejuif to Montlhery, 
the speed of the wind being about one foot per second, or, roughly, three quarters 
