16 DISCUSSIONS IN CLIMATOLOGY. 



of precipitation in the form of snow in temperate 

 regions is at the same time enormously increased by 

 the excess of the evaporation in low latitudes result- 

 ing from the nearness of the sun in perihelion during 

 summer. 



The final result to which we are therefore led is 

 that those warm and cold periods which have alter- 

 nately prevailed during past ages are simply the great 

 secular summers and winters of our globe, depending 

 as truly as the annual ones do upon planetary motions, 

 and like them also fulfilling some important ends in 

 the economy of Nature. 



An important difference. — The physical theory 

 differs from all the preceding in this important respect, 

 viz., that it contains no hypothetical elements. All the 

 causes are real; none hypothetical. The conclusions 

 are all deduced either from known facts, or from 

 admitted physical principles, and in no case are they 

 based on hypotheses. Hypotheses will be found in 

 my cosmological discussions, but none when I deal 

 with climatological questions. 



