TEMPERATURE OF SPACE. 125 



the open air. The mean temperature of space 

 must therefore be lower, and cannot possibly be 

 higher than the lowest observed temperature — 

 namely, -60°C. (76° below zero, Fahr.) 



Let therefore — 60° be assumed to be this point, 

 and let the mean temperatures of three different 

 places on the earth be 0°, 10°, and 20° (Centi- 

 grade), then the numbers, 60, 70, 80 express the 

 proportions of the mean yearly influx of heat 

 into the ground at these three places; we shall 

 soon see that the whole of this heating effect is 

 not due merely to the immediate influence of the 

 sun. 



I have previously mentioned that the variations 

 of temperature, at the surface of the earth, extend 

 to a certain depth in the ground. Hence there 

 must be perceptible daily, monthly, and yearly, 

 highest and lowest temperatures of the parts 

 below as well as of those upon the surface, the 

 differences of which limits for the former are 

 dependent on those of the latter, 



The daily oscillations of the external tempera- 

 ture are, by reason of the slight conductivity of 

 the ground, perceptible only at very moderate 

 depths. The highest temperature of the day is 

 already past before the surface-soil has had time 

 to transmit its heat to the strata below, and this 

 inward progress of the heat is still going on, when 

 the surface has already again begun to cool. 



