Conditions favorable to Glaciation. 99 



The exact value of the greatest possible eccentricity of the 

 earth's orbit is somewhat uncertain. It is dependent upon the 

 masses of the planets, and these are not determined with final 

 accuracy. It is not far from O07 and I shall assume that it is 

 0*0745, the value taken by Sir Eobert Ball. The greatest 

 possible difference between the seasons occurs when the long 

 season is (1 + 4<?/tt) times half the year ; and if X and x are the 

 mean rates of receipt of sunshine during the long and short 

 seasons respectively for greatest eccentricity and greatest differ- 

 ence between seasons in the hemisphere where the winter is 

 long, I find 



X = 1-10788 u 

 X = 0-91590 w-. 



The values are tabulated below with two other sets of values 

 to be explained presently. The diagrams, figures 2 and 3, 

 show the rates graphically, but before commenting upon the 

 differences between the curves it is desirable to consider how 

 the present distribution of heat-rates is related to climate as 

 known by observation. 



No one doubts that temperature is dependent in some man- 

 ner upon solar radiation, but the phenomena are complicated 

 not only by the transfer of heat from one locality to another 

 through the agency of currents of air and water, but also by 

 the selective absorption of the atmosphere, and it is a question 

 therefore how far the mere receipt of sunshine, or what 

 Humboldt called "the astronomical climate," can be made to 

 explain actual climate. That absorption of radiant energy by 

 the atmosphere affects climate has long been understood. 

 Energy of different wave lengths however is differently 

 absorbed by the same gas, and different gases absorb energy 

 differently ; so that the subject is one of great complexity. 

 According to Prof. S. P. Langley,* the temperature of an air- 

 less planet, even under a vertical sun, would be little above the 

 freezing point of water ; and of course the unilluminated part 

 of such a planet would tend toward the absolute zero. Hence 

 the actual temperature of the earth is determined to a very 

 great extent by selective absorption of the atmosphere. No 

 doubt this has always been the case, and, since the composition 

 of the air must have changed as geological time progressed, it 

 is highly probable that differences in selective absorption have 

 determined, or partly determined the differences in mean tem- 



* The Temperature of the Moon. Nat Acad of Sci , vol. iv, part II. 18S9, page 

 193. The conclusion drawn from observations on Mt. Whitney, that an airless 

 planet would fall far short of zero C, is modified in this passage. 



