538 Prof. O. Reynolds on the. 



destroyed. These diverging waves are in their turn destroyed ; so that 

 there is a gradual weakening of the intensity of the waves near the 

 ground, and this weakening extends upwards as the waves proceed. 

 Therefore, under ordinary circumstances, when there is no wind the 

 distant sounds which pass above us are more intense than those which 

 we hear. Of this fact I have abundant evidence. On the 6th, when the 

 wind was light, at all distances greater than 20 yards from the bell the 

 sound was much less at the ground than a few feet above it ; and I was 

 able to recover the sound after it had been lost in every direction by 

 mounting on to a tree, and even more definitely by raising the bell on to 

 a post 4 feet high, which had the effect of doubling the range of the 

 sound in every direction except with the wind, although even in this the 

 range was materially increased. 



It is obvious that the rate at which the sound is destroyed by the 

 ground will depend on the roughness of its surface. Over grass we 

 might expect the sound at the ground to be annihilated, whereas over 

 water it would hardly be affected. This was shown to be the case by 

 the difference in the range at right angles to the wind over grass, and 

 over the same ground when completely covered with snow. In the 

 latter case I could hear the sound at 200 yards, whereas I could only 

 hear it at 70 or 80 in the former. 



Now, owing to the fact that the sound is greater over our heads than 

 at the ground, any thing which slowly brings down the sound will 

 increase the range. Hence, assuming that the action of the wind is to 

 bring down the sound in the direction in which it is blowing, we see 

 that it must increase its range in this direction. And it must also be 

 seen that in this direction there will be less difference in the intensity of 

 the sound from the ground upwards than in other directions. This 

 was observed to be the case on all occasions. In the direction of the 

 wind, when it was strong, the sound could be heard as well with the head 

 on the ground as when raised, even when in a hollow with the bell hidden 

 from view by the slope of the ground ; and no advantage whatever was 

 gained either by ascending to an elevation or raising the bell. Thus, 

 with the wind over the grass the sound could be heard 140 yards, and 

 over snow 360 yards, either with the head lifted or on the ground ; 

 whereas at right angles to the wind on all occasions the range was 

 extended by raising either the observer or the bell. 



It has been necessary to notice these points ; for, as will be 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 



