1 82 Climate 



half-way between the two extreme points to which the 

 thermometer rises and falls. 



But this fact, that the depth at which the tempera- 

 ture of the soil remains always the same is so much 

 greater in one case than in the other, has much influ- 

 ence upon the two climates ; and for this reason : 

 whenever a hot body is in contact with a cold, or cooler 

 one, it at once gives up some of its heat to this other, 

 and continues to do so until there is no difference be- 

 tween the two. 



When, therefore, the sun warms the surface of the 

 soil, the upper layer parts with some of its heat to the 

 one below it, this to the next below, and so on, until 

 that depth is reached where the temperature is always 

 the same. At the equator, therefore, the heat, having 

 only four or five feet of soil to travel through, soon 

 raises the temperature of the whole mass equally, and 

 then, as it cannot descend any lower, it goes on adding 

 to the heat of these upper layers, in which it accumu- 

 lates. At night, when the sun is gone, the surface of 

 the soil cools, and the reverse process 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 



