MEAN TEMPERATURE. 137 



earth, and of the lowest stratum of air that rests 

 immediately upon it, varies between the limits 

 -ISCand +29(0° and 84° Fahr.) It diminishes 

 generally, as of course you are aware, as the 

 latitude increases. 



If we consider the quantity of heat that falls 

 vertically on the equatorial regions of the earth, and 

 the heat thus conferred upon the ground, over and 

 above the temperature of space, — if we consider 

 these as given, then the immediate influence of the 

 sun's heat may be found by calculation from the 

 altitude of the sun, in higher latitudes. Accord- 

 ing to tliis calculation, even assuming the tem- 

 perature of space as not lower than — 60° Centi- 

 grade (i. e., — 76 Fahr.) the mean temperature 

 of the year — at, for instance, the 50th degree of 

 latitude — would still be considerably below 0°, 

 while, however, in reality under this parallel in 

 Germany, it is very nearly as high as + 10° C. 

 ( + 50 Fahr.) Under the arctic circle, the annual 

 mean could not rise above — 50° C. (—60 

 Fahr.) ; and yet, hitherto, nowhere on the earth 

 has a temperature lower than — 56°'7 O. ( — 70 

 Fahr.), and that only transiently, been observed. 

 From this we learn that the warmth of the earth 

 in higher latitudes must be far from being depen- 

 dent exclusively on the immediate influence of the 

 sun. In fact there are many conditions and 

 processes going on upon the surface of the earth, 



