214 THE WEATHER. 



into the atmosphere, directly* towards the sun, it would 

 grow warmer the farther he proceeds. It is not, how- 

 ever, the fact. Whether he mounts in a balloon or 

 ascends a mountain, he finds the cold increases as he 

 leaves the surface of the earth, until he arrives at a region 

 of perpetual ice and snow. The cause is this. 



Suppose we take a sponge, compress it slightly with 

 the hand, and pour upon it as much water as it will 

 receive. While it is now dripping with the surplus, 

 release the pressure. The pores will immediately open, 

 and they will be more than sufficient to absorb the water, ' 

 so that the sponge, in its expanded state, will be com- 



ratively empty. So if the experiment is reversed. 

 a sponge is moderately wet, while in its expanded 

 state, and afterwards compressed with the hand, it will 

 be found that it cannot retain the water which at first it 

 received. 



Now the air is this sponge, compressed near the 

 surface of the earth by the load of atmosphere which is 

 above it ; and the heat which it contains is represented 

 by the water, which was abundant in the compressed, and 

 scarcely perceptible in the expanded sponge. If a por- 

 tion of this air, thus compressed at the surface of the 

 earth, and containing an abundance of heat, rises, it 

 expands, and occupies a larger and larger space : the 

 quantity of heat, therefore, which it contains, is diffused 

 over a greater space, and consequently will produce less 

 sensible effects. In the same manner, if a portion of the 

 air, in an elevated region, having a moderate quantity of 

 heat diffused throughout it, should descend, it will become 

 compressed as it approaches the surface, and the heat 

 which it contains, which before occupied a great space, 

 will now be condensed into a small one, and will produce 

 more sensible effects. In other words, the air will 

 become warmer. These motions of the air, ascending 

 and decsending, are continually taking place, and they 

 keep up a constant difference in temperature of the 

 higher and lower regions. 



Arnott, in his interesting work on Physical Philosophy, 

 makes the following remarks : 'Persons not understand- 

 ing the law which we are now illustrating, will express 

 surprise that wind or air blowing down upon them from a 



