the Refraction of Sound by the Atmosphere. 67 



Which, falling from a considerable height, made very definite waves. 

 Fig. 2 represents a sketch of these waves, made on the spot : the 

 diverging waves from the ends of the direct waves, and also the 



Pig. 2. 



bands of interference, are very 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 

 elevation 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 at- 

 tention. 



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

 containing a battery. The bell was placed horizontally on the top 

 of the case, so that it could be heard equally well in all direc- 

 tions ; 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 11th of March. On the first of these the wind 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 interest 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 assist- 

 ance of Mr. J. B. Millar, of Owens College, whose ears were 

 rather better than mine, although I am not aware of any defi- 

 ciency 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 experi- 

 menting, 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 



* Annates dc Ckimie, vol. i. p. 177 (1816), 



F2 



