THE EEASOK WHY. 63 



"As for the earth, out of it cometh hread ; and under it is turned up as it wer 

 fire." JOB xxvm. 



solar heat, entering our atmosphere at its most attenuated limit 

 (a height supposed to be about fifty miles), would, in passing through 

 the atmosphere to the earth, part with only one-third of its calorific 

 clement. 



222. Wliat lecomes of the remaining two-thirds of the 

 solar heat ? 



They are absorbed, chiefly by the earth, the great medium of 

 calorific absorption; but some portions are taken up by living 

 things, both animal and vegetable. When the rays of heat strike 

 upon the earth's surface, they are passed from particle to particle 

 into the interior of the earth's crust. Other portions are distributed 

 through the air and water by convection, and a third portion is 

 thrown back into space by ridiation. These latter phenomena 

 will be duly explained as we proceed. 



223. How do we know that heat is absorbed, and conducted 

 into the internal earth ? 



It is found that there is a given depth beneath the surface of the 

 globe at which an equal temperature prevails. The depth" increases 

 as we travel south or north from the equator, and corresponds with 

 the shape of the earth's surface, sinking under the valleys, and 

 rising under the hills. 



224. Why may we not understand that this internal heat 

 of the earth arises, as has been supposed by many philosophers, 

 from internal combustion ? 



Because recent investigations have thrown considerable and 

 satisfactory light upon the subject. It has been ascertained that 

 the internal temperature of the earth increases to a certain depth, 

 one degree in every fifty feet. But that below that depth tho 

 temperature begins to decline, and continues to do so with, ever/ 

 increase of depth. 



225. Do plants absorb heat? 



Yes. They both absorb and radiate heat, under varying circum- 

 tances. The majestic tree, the meek flower, the unpretending 

 grass, all perform a part in the grand alchemy of nature. 



