532 Prof. O. Reynolds on the 



I have to say on the sscond part, must be my excuse for introducing it 

 here. In the second part of the subject I have dealt with the effect of 

 the atmosphere to refract sound upwards, an effect which is due to the 

 variation of temperature, and which I believe has not hitherto been 

 noticed. I have been able to show that this refraction explains the 

 well-known difference which exists in the distinctness of sounds by day 

 and by night, as well as other differences in the transmission of sound 

 arising out of circumstances such as temperature ; and I have applied 

 it in particular to explain the very definite results obtained by Professor 

 Tyndall in his experiments off the South Foreland. 



The Effect of Wind upon Sound 



is a matter of common observation. Cases have been known in which, 

 against a high wind, guns could not be heard at a distance of 550 yards*, 

 although on a quiet day the same guns might be heard from ten to 

 twenty miles. And it is not only with high winds that the effect upon 

 sound is apparent ; every sportsman knows how important it is to enter 

 the field on the lee side even when the wind is very light. In light 

 winds, however, the effect is not so certain as in high winds ; and (at any 

 rate so far as our ears are concerned) sounds from a small distance 

 seem, at times to be rather intensified than diminished against very light 

 winds. On all occasions the effect of wind seems to be rather against 

 distance than against distinctness. Sounds heard to windward are for 

 the most part heard with their full distinctness; and there is only a 

 comparatively small margin between that point at which the sound is 

 perceptibly diminished and that at which it ceases to be audible. 



That sound should be blown back by a high wind does not at first 

 sight appear to be unreasonable. Sound is known to travel forward 

 through or on the airland if the air is itself in motion, moving back- 

 wards, it will carry the sound with it, and so retard its forward motion 

 just as the current of a river retards the motion of ships moving up the 

 stream. A little consideration, however, serves to show that the effect 

 of wind on sound cannot be explained in this way. The velocity of 

 sound (1100 feet per second) is so great compared with that of the 

 highest wind (50 to 100 feet per second), that the mere retardation of the 

 velocity, if that were all, would not be apparent. The sound would 

 proceed against the wind with a slightly diminished velocity, at least 

 1000 feet per second, and with a but very slightly diminished intensity. 



Neither can the effect of wind be solely due to its effect on our hear- 

 ing. There can be no doubt that during a high wind our power of 

 hearing is damaged; but this is the same from whatever direction the 

 sound may come ; and hence from this cause the wind would dimmish 

 the distance at which sounds could be heard, whether they moved with 

 it or against it, whereas this is most distinctly not the case. Sounds at 

 * Proc. Boy. Soc. 1874, p. 62. 



