322 
PROFESSOR OSBORNE REYNOLDS ON THE 
These differences in the conditions of the guns and the observers would seem to 
afford good reason why the gnns from Montlhery should have been better heard than 
those at Villejuif, supposing other conditions for the transmission of sound to be equally 
favourable both ways ; but the wind was blowing from Villejuif to Montlhery, and that 
this should not have reversed the effect is the most remarkable part of the phenomenon. 
This is remarkable, however, only on the supposition that the effect of the wind upon 
sound is invariable. As it seemed to me that there were several good reasons for 
supposing that this is not the case, I thought it might be worth while trying a few 
observations. I accordingly made some experiments with my electric bell on some very 
calm nights in May and June, with the following results : — 
When the sky was cloudy and there was no dew, the sound could invariably be heard 
much further with the wind than against it, even when the wind was not more than one 
foot per second. 
But when the sky was clear and there was a heavy dew, the sound could be heard as 
far against a light wind as with it, and sometimes much further. On one occasion, when 
the wind was very light (about 1 foot per second at 6 feet above the ground) and the 
thermometer showed 39 degrees at 1 foot above the grass and 47 at 8 feet, the sound 
was heard at 440 yards against the wind, and 270 yards with it. 
Now the nights on which Arago made his experiments were clear ; there was a heavy 
dew, and the thermometer at Montlhery showed that at that elevation the temperature 
was 2° F. greater than at Villejuif; so that after the experiments just described there is 
nothing surprising in the fact that the wind did not produce much effect on the 
sound. 
A good reason (as I have previously stated) may be given in explanation of these 
changes in the effects of the wind. The wind tends to lift the sound proceeding against 
it and to bring down that which is travelling with it. These effects are greatest near 
the earth and diminish as we proceed upwards (for the simple reason that the retarda- 
tion of the wind is greater near the surface). The effect of the wind, therefore, will be 
to intensify the sound proceeding against it at sufficiently high elevations (this was 
found to be the case in my first experiments) and to weaken the sounds proceeding with 
it at points at some height above the surface — that is, when the sound which is brought 
rough surface is a most effectual barrier to sound — sound produced close to one side of the stack being quite 
inaudible on the other side. On this occasion, however, I found the most perfect reciprocity ; sounds produced 
close behind the stack could he heard at a distance just as well, and no better, than similar sounds at a distance 
could be heard behind the stack, provided always that great care was taken to bring the ear behind the stack 
into exactly the same position as that previously occupied by the source of sound. It appears, however, that a 
few inches difference in the position of the ear on the source of sound was sufficient to make all the difference 
as to the audibility of the sound. These experiments therefore, although they confirmed Lord Rayleigh and 
showed my previous idea to have been wrong, suggested another explanation of the phenomenon which had 
led me to it. They show that the apparent absence of reciprocity was in reality caused by my not having 
taken sufficient notice of small difference in the position of the ear and the bell, and they suggest that the 
apparent want of reciprocity in the experiments made at Yillejuif and Montlhery was due in the same way to 
the small differences in the positions of the guns and the ears of the auditors, as pointed out in the text. 
