Climate i r o 



difference between day and night. It is not often that 

 the thermometer stands above 90° or 91° F. by day, or 

 falls below 74° F. by night. And the temperature of the 

 soil varies but little, too. Four or five feet below the 

 surface it never varies at all, and remains constantly at 

 80° F., just about half-way between the temperature of 

 night and day. 



Where the temperature of the air varies more, there 

 the temperature of the soil varies more also, and to a 

 greater depth. At the equator, the sun affects only the 

 upper four or five feet of the soil, the change between day 

 and night being unfelt below this; but in England, the 

 change between summer and winter is felt to a depth of 

 fifty or sixty feet, probably; and below this the temperature 

 remains steady at but a little above 40° F., which is about 

 the mean temperature of the air in England, as 80° F. is 

 that of the equatorial region — half-way between the two 

 extreme points to which the thermometer rises and falls. 



But this fact, that the depth at which the temperature 

 of the soil remains always the same is so much greater in 

 one case than in the other, has much influence 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 between 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 



