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The Great World's Farm 



any lower, it goes on adding to the heat of these upper 

 layers, in which it accumulates. At night, when the sun 

 is gone, the surface of the soil cools, and the reverse pro- 

 cess begins; the heat stored during the day gradually 

 passes up again to the surface, and from the surface into 

 the air, so that both earth and air are kept at a more even 

 temperature than would otherwise be the case. 



But where, as in England, the sun has forty or fifty 

 feet of cool earth to warm, naturally it is much longer 

 about it, and the whole mass is consequently not warmed 

 equally through till summer is at its height. Then, and 

 not till then, the whole mass being warm, heat begins to 

 be stored during the day in the upper layers, and is given 

 up again, when the sun is down, to warm the air at night. 

 Hence we have warm nights in July and August; and 

 winter is less cold than it would otherwise be, thanks 

 to the heat gradually given back by the great mass of 

 earth. 



But the heat given back to the air in this way would 

 quickly escape, and benefit us little, were it not for the 

 moisture in the air, which acts the part of a blanket, and 

 keeps it near the earth. At the equator, the air is gen- 

 erally in a very moist condition all the year round; and this 

 moisture, while serving the part of a blanket by night, 

 acts as a veil or parasol by day, and prevents the sun's 

 rays from being too scorching. 



It is to the large quantity of moisture in the air that 

 the region of the equator owes its very even temperature, 

 therefore. 



In the dry plains of north India the heat is scorching, 

 much greater than at the equator, just for want of this 

 veil ; and the nights are often so cold that water is frozen, 



