COLD AND ICE ON THE EARTH 



below freezing, it is several hundreds of degrees warmer 

 than the space outside the earth. Midway between 

 the earth and the moon the temperature must be 430 F. 

 below freezing; so that if we take the surface of the 

 earth as a whole we may say that it is between four 

 hundred and five hundred degrees warmer than the space 

 by which it is surrounded. Every one knows what is 

 happening when he warms his hands at a fire. The fire 

 being hotter than its surroundings is giving out heat 

 towards them, and the hands catch some of this radiated 

 heat. Similarly the earth is radiating heat, and the 

 atmosphere round the earth catches some of it. So also 

 do the seas. While therefore it is certain that the heat 

 of the sun warms the earth and the air and the sea, and 

 so gives rise to currents of air and perhaps of water, so 

 also is it likely that the heat of the earth causes warm 

 air to rise, and so plays its part in forming the winds, 

 the currents of air, and the currents of water. When 

 the earth was warmer than it is now it had more and 

 greater effects in this direction. It caused more evapora- 

 tion of the water, more clouds, and therefore more rain, 

 and in winter more snow. 



Suppose, then, a period when there was very much 

 more snow in winter than now. As the snow accumulated 

 layer on layer the lower part would become squeezed into a 

 mass half ice, half snow ; and it is quite likely that the 

 heat of the summer sun (especially if, as we have supposed, 

 the atmosphere was much cloudier then than now) would 

 be unable to dissolve it. Thus the snow age would 

 gradually merge into an ice age, and we can imagine a 



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