28 PROCEEDINGS OF THE AMERICAN ACADEMY 



If a permanent increase should take place in the amouut of solar 

 radiation, it is clear, upon the present hypothesis, that some time would 

 elapse before the increasing agitation of the atmosphere put an end to 

 the increase of its energy. The stock of atmospheric energy in gen- 

 eral, and that part of it manifested as heat, would thus be permanently 

 increased. The terrestrial temperature would be raised, tempests 

 would be more frequent and severe, and the entire atmosphere would 

 probably occupy more space. 



On the other hand, a permanent diminution in solar radiation would 

 tend to diminish the agitation of the air, and, although the terrestrial 

 temperature would decline, this loss of temperature would not be so 

 great as that which would have occurred if the winds had maintained 

 their previous force. The atmosphere, accordingly, acts as a check 

 upon extreme variations of heat and cold ; when little heat is received, 

 it will be better economized than when the supply of heat is excessive, 

 although particular regions may have, in the former case, a very 

 severe climate. 



The observed association of extreme cold with still air, and the 

 greater violence of tempests in the heated portions of the world, on the 

 whole, are among the facts tending to support the hypothesis above 

 explained. 



In the present discussion, the consequences resulting from the com- 

 pressibility of the atmosphere have thus far been neglected, and what 

 has been said above would be equally applicable to an atmosphere 

 wholly incompressible. But in such an atmosphere the distribution of 

 heat would materially differ from that actually observed. As a con- 

 vection current rises, the air composing it expands, from the removal 

 of pressure, and its energy largely ceases to exhibit itself as heat. 

 Under these circumstances, the solid and liquid particles carried up 

 with it are reduced in temperature, and made less capable of radiation 

 into space than before. It may likewise be supposed, indeed, that the 

 expanded air itself will have its previously small capacity for the dis- 

 charge of its energy into space still further diminished. The addi- 

 tional tendency to retain energy, thus suggested, would demand more 

 consideration if the discharge were effected by any process of the 

 nature of conduction ; that is, if the outer surface of the atmosphere 

 were chiefly instrumental in the process. In this case, the expansion 

 of the ascending air would be a highly important means of delaying 

 the escape of energy received by conduction from warm ground ; 

 and an incompressible atmosphere might not in any considerable 

 degree protect the planet which it surrounded. 



