EEFEACTION OF SOUND BY THE ATMOSPHEEE. 
down is destroyed by the roughness of the surface, though over a calm sea, the sound 
brought down would roll along the surface as in a whispering-gallery. Now when the 
temperature diminishes upwards, as it does generally during a calm day, the effect of 
the refraction thus caused will be to increase the effect of the wind on sound moving 
against it, and to diminish that on the sound moving with it. But when the diminu- 
tion of temperature is downwards, as it was at Villejuif and Montlhery, and as it always 
is near the earth on a clear dewy night, it will directly diminish the effect on sound 
moving against the wind, and increase it on the sound moving with the wind. That is 
to say it will prevent the wind lifting the sound in one direction and will aid it in 
bringing it down in the other. Thus it will prolong the distance to which sound can 
be heard against the wind, and diminish that at which it can be heard with the wind 
(when the surface is rough) ; and when the downward diminution of temperature bears 
a certain relation to the strength of the wind, it is easy to see that it may neutralize or 
even reverse its effect. 
These facts, all taken together, appear to me to afford a satisfactory explanation of the 
phenomenon observed by Arago. There was, however, one other phenomenon observed 
during the same experiments on which I will venture a word in explanation. 
The reports of the guns at Montlhery as heard at that station were attended with 
prolonged echoes, but it was not so with those at Villejuif. This phenomenon was not 
explained by the experimenters ; but I think it admits of a simple explanation. The 
ground surrounding Villejuif towards Montlhery is very flat with not a tree upon it for 
miles, and being all arable would at that time of the year be covered with crops. 
Around Montlhery the country is hilly, some of the hills rising 100 feet above 
Montlhery itself ; their sides are in many places precipitous, and are largely covered 
with trees. From the flat country around Villejuif there would arise no echoes, but 
from the hills and trees around Montlhery it is quite certain that there must arise very 
considerable echoes ; and hence it seems to me that the phenomenon becomes simple 
enough. 
The Report of the American Lighthouse Board. 
I may remark, in conclusion, that I have just received a copy of the Report of the 
American Lighthouse Board, kindly sent me by Dr. Henry, the Chairman of the 
Board. In an Appendix to this Report, Dr. Henry has given an account of his experi- 
ments on the transmission of sound, undertaken for the Board, and extending over the 
last thirty years. These experiments have led him to the conclusion that the differences 
in the distances at which the same sound can be heard at different times are in all 
cases to be explained by refraction. He has ascribed the cause of the refraction to the 
wind ; and to explain cases in which the refraction did not accord with the direction of 
the wind, he points out that it is not sufficient to know the direction of the wind at the 
surface, but that in order to say what would be its effect upon sound, we should know in 
what direction it is blowing above ; for it is not the simple motion of the wind which affects 
sound, but the difference between its motion above and below. This is very true ; and 
