the Refraction of Sound by the Atmosphere. G 3 



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 air ; and if the air is itself in motion, 

 moving backwards, 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, howeA^er, 

 serves to show that the effect of wind on sound cannot be ex- 

 plained 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 dimiuished 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 

 heariug. 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 dimiuish the distance at which sounds could be heard, 

 whether they moved with it or against it, whereas this is most di- 

 stinctly not the case. Sounds at right angles to the wind are but 

 little affected by it ; and in moderate winds sounds can be heard 

 further with the wind than when there is none. 



The same may be said against theories which would explain the 

 effect of wind as causing a heterogeneous nature in the air so 

 that it might reflect the sound. All such effects must apply with 

 equal force with and against the wind. 



This question has baffled investigators for so long a time because 

 they have looked for the cause in some direct effect of the motion 

 of the air, whereas it seems to be but incidentally due to this. The 

 effect appears, after all, not to be due simply to the wind, but to 

 the difference in the velocity with which the air travels at the sur- 

 face of the ground and at a height above it. That is to say, if we 

 could have a perfectly smooth surface which would not retard the 

 wind at all, then the wind would not obstruct sound in the way it 

 does, for it would all be moving with an equal velocity ; but, owing 

 to the roughness of the surface and the obstructions upon it, there 

 is a gradual diminution in the velocity of the wind as it approaches 

 the surface. The rate of this diminution will depend on the na- 

 ture of the surface ; for instance, in a meadow the velocity at 1 

 foot above the surface is only half what it is at an elevation 61; 



