120 heat or earth's outer crust; 



greatest precisely at this time. The temperature 

 of the earth must therefore continue to rise for 

 some time after mid-day, or, in other words, the 

 greatest diurnal warmth is always some time after 

 noon, — in summer about three o'clock, in winter 

 about one p.m. For a short time now the receipt 

 and expenditure of heat are equal ; then the latter 

 preponderates, and cooling begins, at first slowly, 

 since a part of the loss is continually restored by 

 the presence of the sun, more quickly after sunset. 

 All night through the temperature falls, and even 

 some time after the sun is up, for the first in- 

 fluence of the early morning sun is not sufficient 

 to counterbalance the loss of heat that is still going 

 on. At last the equilibrium is attained, about 

 half an hour after sunrise. This is the point of 

 lowest temperature. 



The lower strata of the earth's crust, sharing as 

 they do in the heat received by the surface, must 

 in turn, from the store they have before acquired, 

 pay back in temperature to it when the cool- 

 ing of the latter is proceeding. Por this reason 

 the cooling process goes on more slowly than, 

 without this agency, would be the case. 



In every place on the earth, where the lengths 

 of the day and night are subject to but slight 

 variation during the course of the year, these alter- 

 nations of the temperature of the ground must be 

 repeated from day to day in nearly the same manner. 



