72 Royal Society : — Prof. Reynolds on 



the phenomena under consideration ; for it gives to the air a much 

 greater power of radiating and absorbing heat, and thus renders 

 it much more susceptible of changes in the action of the sun. 



If, then, the air were all at the same temperature and equally sa- 

 turated with moisture, the velocity of sound would be the same at 

 all elevations; but if the temperature is greater, or if it contains 

 more water below than above, then the wave of sound will proceed 

 quicker below than above, and will be turned up in the same way 

 as against a wind. This action of the atmosphere is, strictly speak- 

 ing, analogous to the refraction of light. In light, however, it is 

 density which retards motion ; temperature and pressure have little 

 or nothing to do with it; and since the density increases downwards, 

 the rays of light move slower below than they do above, and are 

 therefore bent downwards, and thus the distance at which we can 

 see objects is increased. "With sound, however, since it is tempera- 

 ture which affects the velocity, the reverse is the case ; the rays 

 are bent upwards, and the distance from which we can hear is re- 

 duced. 



It is a well-known fact that the temperature of the air diminishes 

 as we proceed upwards, and that it also contains less vapour. 

 Hence it follows that, as a rule, the waves of sound must travel 

 faster below than they do above, and thus be refracted or turned 

 upward. 



The variation of temperature is, however, by no means constant, 

 and a little consideration serves to show that it will be greatest 

 in a quiet atmosphere when the sun is shining. The sun's rays, 

 acting most powerfully on that air which contains the most vapour, 

 warms the lower strata more than those above them ; and besides 

 this, they warm the surface of the earth, and this warmth is taken 

 up by the air in contact with it. It is not, however, only on such 

 considerations as these that we are in a position to assert the law 

 of variation of atmospheric temperature. Mr. Grlaisher has fur- 

 nished us with information on the subject which places it beyond 

 the region of surmise. 



I extract the following from his "Keport on Eight Balloon 

 Ascents in 1862" (Brit. Assoc. Eep. 1862, p. 462) :— 



" From these results the decline of temperature when the sky 

 was cloudy 



For the first 300 feet was 0°«5 for every 100 feet. 

 From 300 to 3400 „ 0°-4 

 „ 3400 to 5000 „ 0°-3 



" Therefore in cloudy states of the sky the temperature of the 

 air decreased nearly uniformly with the height above the surface of 

 the earth nearly up to the cloud. 



" When the sky was partially cloudy the decline of temperature 

 In the first 100 feet was 0°-9 



* * * * 



From 2900 to 5000 „ 0°-3 for every 100 feet. 



" The decline of temperature near the earth with a partially clear 

 sky is nearly double that with a cloudy sky. 



" In some cases, as on July &0th, the decline of temperature in 

 the first 100 feet was as large as 1 0, 1." 



