the Refraction of Sound by the Atmosphere. 69 



seen, they bear directly on the question of the effect of elevation 

 on the range of sound against the wind. 



Elevation was found to affect the range of sound against the 

 wind in a much more marked manner than at right angles. 



Over the grass no sound could be heard with the head on the 

 ground at 20 yards from the bell, and at 30 yards it was lost with 

 the head 3 feet from the ground, and its full intensity was lost when 

 standing erect at 30 yards. At 70 yards, when standing erect, the 

 sound was lost at long intervals, and was only faintly heard even 

 then ; but it became continuous again when the ear was raised 9 

 feet from the ground, and it reached its full intensity at an eleva- 

 tion of 12 feet. 



Over the snow similar effects were observed at very nearly 

 equal distances. There was this difference, however, the sound 

 was not entirely lost when the head was lowered or even on the 

 ground. Thus at 30 yards I could still hear a faint sound. Mr. 

 Millar could hear this better than I could ; he, however, experi- 

 enced the same increase on raising his head. At 90 yards I lost 

 the sound entirely when standing on the ground, but recovered it 

 again when the ear was 9 feet from the ground. Mr. Millar, how- 

 ever, could hear the sound very faintly, and at intervals, at 160 

 yards ; but not with his head on the ground. At this point I was 

 utterly unable to hear it ; and even at an elevation of 25 feet I 

 gave it up as hopeless. However, as Mr. Millar by mounting 10 

 feet higher seemed to hear it very much better, I again ascended ; 

 and at an elevation of 33 feet from the ground I could hear it 

 as distinctly as I had previously heard it when standing at 90 

 yards from the bell. I could not hear it 5 feet lower down ; so 

 that it was the last 5 feet which had brought me into the foot of 

 the wave. Mr. Millar experienced the same change in this 5 feet. 

 As the sound could now be heard as strong as at a corresponding 

 distance with the wind, we thought we had reached the full inten- 

 sity of the waves. This, however, was not the case ; for the least 

 raising of the bell was followed by a considerable intensifying of 

 the sound ; and when it was raised 6 feet I could hear each blow of 

 the hammer distinctly, although just at that time a brass band was 

 playing in the distance. It seemed to me that I could hear it as 

 distinctly as at 30 yards to leeward of the bell. All these results 

 were repeated on both days with great uniformity. 



When more than 30 yards to the windward of the bell, the 

 raising of the bell was always accompanied by a marked intensi- 

 fying of the sound, and particularly over the grass. I could only 

 hear the bell at 70 yards when on the ground ; yet when set on 

 a post 5 feet high I heard it 160 yards, or more than twice the 

 distance. This is a proof of what I previously pointed out, that 

 the waves rise faster at the ground than they do high up, and 

 crowding together they intensify. In all cases there was an unmis- 

 takable greater distinctness of the sound from short distances of 

 windward than to leeward or at right angles. 

 • Except when the sound was heard with full force it was not 

 uniform. The bell gave two sounds (the beats of the hammer and 

 the ring) which could be easily distinguished ; and at times we 



