68 Royal Society : — Prof. Reynolds on 



a break in the continuity of sound, a stoppage of the bell. The 

 difference in our method of experimenting accounts for the differ- 

 ence 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 experiments, is that the sound which comes in immediate con- 

 tact 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 destroyed. These diverging waves are in their 

 turn destroyed ; so that there is a gradual weakening of the in- 

 tensity of the waves near the ground, and this weakening extends 

 upwards as the waves proceed. Therefore, under ordinary circum- 

 stances, 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 direc- 

 tion. 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 what- 

 ever 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 



