Refraction of Sound by the Atmosphere. 537 



clearly seen. Both these figures agree with what has been explained 

 as the effect of wind on sound. 



In the next place I endeavoured to ascertain the effect which eleva- 

 tion has on the distance to which sound can be heard against a wind. 

 In making these experiments I discovered some facts relating to the 

 transmission of sound over a rough surface, which, although somewhat 

 obvious, appear hitherto to have escaped attention. 



My apparatus consisted of an electrical bell, mounted on a case con- 

 taining a battery. The bell was placed horizontally on the top of the 

 case, so that it could be heard equally well in all directions ; and when 

 standing on the ground the bell was 1 foot above the surface. I also 

 used an anemometer. 



These experiments were made on four different days, the 6th, 9th, 

 10th, and llth of March. On the first of these the w T ind was very light, 

 on the others it was moderately strong, strongest on the second and 

 fourth ; on all four the direction was the same, viz. north. On the two 

 last days the ground was covered with snow, which gave additional in- 

 terest to the experiments, inasmuch as it enabled me to compare the 

 effect of different surfaces. On the first two days I. was alone, but on 

 the last two I had the assistance of Mr. J. B. Millar, of Owens College, 

 whose ears were rather better than mine, although I am not aware of 

 any deficiency in this respect. The experiments were all made in the 

 same place, a flat meadow of considerable extent. 



The General Results of the Experiments. 



De La Roche*, in his experiment, found that the wind produced least 

 effect on the sound at right angles to its direction, i. e. sounds could be 

 heard furthest in this direction. His method of experimenting, however, 

 was not the same as mine. He compared the sounds from two equal 

 bells, and in all cases placed the bells at such distances that the sounds 

 were equally distinct. I, on the other hand, measured the extreme 

 distance at which the sounds could be heard, the test being whether or 

 not the observer noticed a break in the continuity of sound, a stoppage 

 of the bell. The difference in our method of experimenting accounts for 

 the difference in our results. I found in every case that the sound could 

 be heard further with the wind than at right angles to its direction ; and 

 when the wind was at all strong, the range with the wind was more than 

 double that at right angles. It does not follow, however, nor was the 

 fact observed, that at comparatively short distances the sound with the 

 wind was more intense than at right angles. 



The explanation of this fact, which was fully borne out by all the ex- 

 periments, is that the sound which comes in immediate contact with the 

 ground is continually destroyed by the rough surface, and the sound from 

 above is continually diverging down to replace that which has been 

 * Annales de Chimie, vol. i. p. 177 (1816). 



