REFRACTION OF SOUND BY THE ATMOSPHERE. 
321 
of a mile an hour. This remarkable want of reciprocity was much commented on by 
the observers, although they appear to have been entirely at a loss to account for it. 
On reading M. Aeago’s report*, I noticed that the observations on the barometer 
showed Montlhery to be about 80 feet above Yillejuif, and it occurred to me that 
this difference of elevation might afford a clew to the mystery. I had observed in my 
observations of the effect of wind upon sound that a difference of a few feet in the height 
of the observer or in the source of sound, especially when near the ground, often made 
all the difference between hearing distinctly and not hearing at all. It appeared to me 
probable, therefore, that there might be something advantageous in the situations of the 
gun at Montlhery and the observers at Villejuif over the situations of the gun at Yillejuif 
and the observers at Montlhery. I was confirmed in this impression by a fact mentioned 
by Akago, viz. that on the first night the gun at Yillejuif had been pointed upwards 
at a considerable angle, but that thinking this might have had something to do with its 
not being heard so well as the other, on the second night it was brought down to the 
horizontal. The result, however, was that the gun was not heard so well on the second 
night as it had been on the first. This remark concerning the gun at Yillejuif seemed 
to imply that it was fired from level ground and at no great elevation, whereas at 
Montlhery it seemed possible that the gun might have been fired over a parapet. To 
settle this question I took an opportunity last Easter of walking over the ground from 
Villejuif to Montlhery, and by the aid of a map made a section of it. 
The two stations are visible from each other ; that at Yillejuif is on the top of a 
gently rising hill, whereas that at Montlhery is on the top of a very steep sugar-loaf 
hill, terminating in the mound of an old castle, which is supported on the side facing 
Villejuif by a wall some 20 feet vertical, and then so steep that Villejuif can be seen 
over the tops of the trees surrounding the castle. Part of the old parapet wall is left, 
and it is impossible to believe but that any one firing a gun from that spot would place 
it with its muzzle over the parapet. It seems very probable, therefore, that the gun at 
Montlhery was fired over the parapet, which would be the most favourable position for 
being heard, as the direct sound would be strengthened by that reflected from the wall 
below it, while the observers, standing somewhat behind the parapet, would not have 
the advantage of any reflected sound, and would therefore be in a disadvantageous 
position as compared with the muzzle of the gun. At Yillejuif the case would be 
different ; the gun, as fired’on level ground, would be at a disadvantage compared with 
the observers, whose ears would be considerably above it. That this difference was 
sufficient to affect the results seems to have been proved by the evil effect of lowering 
the muzzle of the gun f . 
* Annales de Chimie, 1822, p. 21J . 
f Erom my previous experiments on the effect of wind upon sound, I had been led to the conclusion that 
under certain circumstances there may be an absence of reciprocity in the passage of sound backwards and 
forwards between two points. Lord Rayieigh, however, pointed out to me that there are strong reasons for 
believing that this is not the case. To prove the force of these reasons, I made some observations behind a 
large wheat-stack standing alone oh level ground, experience having shown me that a wheat-stack from its 
