Refraction of Sound by the Atmosphere. 539 



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 elevation 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, experienced 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, however, 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 intensity 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 intensifying 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 unmistakable greater distinctness of the sound from short distances to 

 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 could hear only the ring, 

 and at others the beats. The ring seemed to preserve itself the longest ; 

 whereas near the ground at short distances the ring was lost first. This 

 is explained by the fact that the rate at which sound-waves diverge 

 depends upon their note : the lower the note the more will they diverge. 



