LETTEB VIII. 



ON THE TEMPERATURE OF THE OUTERMOST CRUST 

 OF THE EARTH. 



Xearly all the heat, that is in the surface of 

 the earth, comes to it from the sun, reaching our 

 planet by radiation. The heat-rays have the pro- 

 perty of imparting warmth only to such bodies as 

 can arrest their motion, or, as we say, absorb them. 

 The air is not a substance of this kind, but, on the 

 contrary, gives free passage to the greatest part of 

 the heat-rays, without deriving warmth from them, 

 just as it is traversed by the light; and thus it is 

 that, although, in their path to the earth, the rays 

 of the sun must pass tlirough the atmosphere, yet 

 they do not part with any considerable portion of 

 then heat, until they arrive on the solid surface £ 

 The exact analysis of this behaviour is one of the 

 most interesting tasks of experimental physics ; but 

 the simple fact can scarcely escape even an unprac- 

 tised observer. I need only remind you of the 

 impression you receive on passing suddenly from 

 the shade into bright sunshine, on removing a 

 screen from between your face and a glowing fire, 

 or on coming suddenly upon a close view of a 

 smith's forge. You know that in all such cases 



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