542 Prof. O. Reynolds on the 



and every degree of temperature between 32 and 70 adds approximately 

 1 foot per second to the velocity of sound. This velocity also increases 

 with the quantity of moisture in the air ; but the quantity is at all times 

 too small to produce an appreciable result. This vapour nevertheless 

 plays an important part in 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 saturated 

 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 speaking, 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 tempe- 

 rature which affects the velocity, the reverse is the case ; the rays are 

 bent upwards, and the distance from which we can hear is reduced. 



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 sur- 

 face 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 furnished us with information on the subject which 

 places it beyond the region of surmise. 



I extract the following from his "Report on Eight Balloon Ascents in 

 1862" (Brit. Assoc. Rep. 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. 



