124 heat op earth's outer crust. 



It is evident that places of equal mean tempera- 

 ture inust, on the average, receive equal quantities 

 of heat, and that any district will be the more 

 powerfully heated the higher the value of its mean 

 temperature. These mean numbers, however, 

 taken alone, by no means indicate the proportions 

 of heat taken in at different places, since we must 

 know, besides this, the temperature of the surround- 

 ing space, or the degree of heat from which the 

 warming effect of the sun's rays commences, and 

 below which the surface of the earth could not be 

 cooled even during total absence of the sun. 



I have already reminded you that the sun is not 

 the only heavenly body from which the earth de- 

 rives warmth. The fixed stars make some, though 

 but very small, contributions. If the sun were not 

 there, the earth would be cooled down, until the 

 continual escape was in equilibrium with the 

 ingress of heat from the starry heavens. You will 

 see that there would result a certain degree of 

 heat, equally distributed over all parts of the 

 earth; this is what we call the Temperature of 

 Space. Fourier, who was the first to call atten- 

 tion to the lowest possible depression, towards this 

 extreme limit, of the temperature of any spot on 

 the earth, believed that— 45° Centigrade (49° below 

 zero, on Fahrenheit's scale) should be taken for 

 this point. But since his time, degrees of cold 

 considerably lower than this have been observed in 



