ON THE SUN'S HEAT. 397 



from the sun on its way to the earth. In the case 

 of the sun the heat radiated from the outer parts 

 of the atmosphere is wholly derived from the 

 interior. In both cases the whole fluid mass is 

 kept thoroughly mixed by currents of cooled fluid 

 coming down, and of warmer fluid rising to take 

 its place and to be cooled and descend in its turn. 



Now it is a well-known property of gases and 

 of fluids generally (except some special cases, as 

 that of water within a few degrees of its freezing 

 temperature, in which the fluid under constant 

 pressure contracts with rise of temperature) that 

 condensations and rarefactions, effected by aug- 

 mentations and diminutions of pressure from 

 without, produce elevations and lowerings of 

 temperature in circumstances in which the gas is 

 prevented from either taking heat from or giving 

 heat to any material external to it. Thus a 

 quantity of air or other gas taken at ordinary 

 temperature (say 15 C. or 59 F.) and expanded 

 to double its bulk becomes 71 C. cooler; and if 

 the expansion is continued to thirty-two times 

 its original bulk it becomes cooled 148 further, 



