ORIGIN OF THE SUN's HEAT. 281 



ature, is of the greatest importance to mankind. If the solar 

 heat were originated by a chemical process, and amounted 

 near its source to a temperature of a few thousand degrees, 

 it would be possible for the light to reach us, whilst the 

 greater part of the more important calorific rays would be ab- 

 sorbed by the higher strata of our atmosphere and then re- 

 turned to the universe. 



In consequence of the high temperature of the sun, how- 

 ever, our atmosphere is highly diathermic to his rays, so that 

 the latter reach the surface of our earth and warm it. The 

 comparatively low temperature of the terrestrial surface is 

 the cause why the heat cannot easily radiate back through the 

 atmosphere into the universe. The atmosphere acts, there- 

 fore, like an envelope, which is easily pierced by the solar 

 rays, but which offers considerable resistance to the radiant 

 heat escaping from our earth ; its action resembles that of a 

 valve which allows liquid to pass freely in one, but stops the 

 flow in the opposite direction. 



The action of the atmosphere is of the greatest impor- 

 tance as regards climate and meteorological processes. It 

 must raise the mean temperature of the earth's surface. Af- 

 ter the setting of the sun in fact, in all places where his 

 rays do not reach the surface, the temperature of the earth 

 would soon be as low as that of the universe, if the atmos- 

 phere were removed, or if it did not exist. Even the power- 

 ful solar rays in the tropics would be unable to preserve wa- 

 ter in its liquid state. 



Between the great cold which would reign at all times 

 and in all places, and the moderate warmth which in reality 

 exists on our globe, intermediate temperatures may be ima- 

 gined ; and it is easily seen that the mean temperature would 

 decrease if the atmosphere were to become more and more 

 rare. Such a rarefaction of a valve-like acting atmosphere 

 actually takes place as we ascend higher and higher above 



