THE SURFACE OF THE EARTH. 



from the countless myriads of suns composing the stellar universe 

 which are seen nightly sparkling in the heavens ; and this latter 

 source of heat differs from that of the sun, inasmuch as all parts 

 of the earth are constantly exposed to it night and day, whereas 

 they are only exposed to the direct influence of the sun during 

 average intervals of twelve hours. 



It has resulted from the experiments and observations of 

 M. Pouillet, that the quantity of heat thus received by the earth 

 from the celestial spaces is such, that if it were uniformly diffused 

 over the surface it would be sufficient to. melt a shell of ice, 

 enveloping the earth, 85 feet thick.* 



183. "Why temperature of the earth is not indefinitely 

 increased. It may be asked, therefore, how it happens that if 

 the earth receive and absorb an annual quantity of heat so 

 enormous, its temperature is not raised to such a point as to be 

 incompatible with the continued existence of the organised world 

 upon it. It might be expected that the heat absorbed in the 

 torrid zone, being transmitted by the process of conduction 

 through the solid matter composing the earth to the colder regions 

 of the poles, would first dissolve all the ice there collected, and 

 then gradually raise the temperature of the polar regions, to 

 which the animal world would first take refuge flying from the 

 scorching region of the tropics, and which would become at the 

 same time the theatre of the present flora of the temperate and 

 tropical zones, and later would be raised to a temperature 

 destructive of all organisation. 



Such a catastrophe is prevented, and the thermal condition of the 

 globe maintained within the necessary limits, by a remarkable pro- 

 perty with which matter has been endowed by its Maker, in virtue 

 of which all bodies possess the quality of thermal radiation, in the 

 exact proportion in which they are endowed with that of thermal 

 absorption. The heat, therefore, which every part of the surface 

 of the earth absorbs from the solar rays, it throws back by the 

 process of superficial radiation, and if the atmosphere were every- 

 where cloudless, the heat thus radiated back would pass into the 

 celestial spaces, and the atmosphere would then, in spite of the 

 solar rays passing through it, remain at a very low temperature. 



184. Effect of clouds. The existence of clouds, however, 

 modifies this. The heat radiated from the surface of the earth is 

 in a great degree intercepted, and prevented from escaping into the 

 celestial regions, by the clouds, which reflect and radiate it back 

 again to the earth through the lower strata of the atmosphere ; 

 and thus these thermal rays, being reverberated between the 



* See Tr;ict on Terrestrial Heat, Museum, vol. 3, p. 65. 



182 



