484 Rev. E. Sill — On Evaporation and Eccentricity. 



perihelion, then even though the utmost snowfall should have reached 

 hut 20|^ inches, and the unmelted annual increase but half an inch, 

 still the total accumulation would be 250 feet. Even now, snow 

 sometimes lies all the year through on Ben Nevis. Scotland would 

 be very different with such a snow cap on its peaks. 



The fluctuation of 10" assumed above has not been taken quite at 

 random. Maximum eccentricity might produce a fluctuation of over 

 20'^ were radiation the only means of escape for the heat.^ This is 

 too great, for the present eccentricity ought by a like calculation to 

 produce a difference of temperature between January and June 

 amounting to about lO'^ at the Equator. Actually it is, according to 

 K. Johnston, only some 2°. On the other hand (on the same 

 authority), in the Tropics three-fourths of the rainfall occurs in the 

 three hottest months. So it would appear that the additional heat 

 goes almost entirely to augment the precipitation. We can hardly 

 doubt that intenser heat would still further augment the summer 

 precipitation, while that of the remaining months is too small to be 

 very much diminished. For Arctic regions, the few registers I can 

 find (Rink, Nares, Nordenskiold) point either to a similar summer 

 excess, or to a precipitation coming mainly in spring and autumn. 

 A hotter summer would charge the atmosphere with more vapour, 

 which would be precipitated in autumn, probably with more snow. 



This action, whatever be its magnitude, works in harmony with 

 Dr. CroU's views. For the greatest extremes of summer heat and 

 winter cold occur with high eccentricity, and with summer during 

 perihelion. The action ought to be most energetic on land sur- 

 rounded by water. For the summer heat warming the water will 

 produce the intensified evaporation explained, whilst the land will 

 begin to cool faster than the water under winter's cold, and an in- 

 draught of moist air may produce increased precipitation. This 

 agrees with the vast extent of the Antarctic ice cap which has always 

 seemed to me too far different from the Arctic ice fields to be due 

 simply to difference of seasons. 



In conclusion, I may remark that the immobility of water in the 

 form of snow, expounded by Mr. Wallace in "Island Life," and the 

 increased loss when heat is liberated at a greater elevation, sug- 

 gested by Dr. Eoberts in the Geological Magazine, are both 

 essential points in this theory. The present article supplements 

 them by a third point, that as temperature increases, evaporation 

 increases, but many times as fast, in fact with a transcendental ratio. 

 Were it not for this, alterations of eccentricity would produce only 

 mutually compensating effects. Mr. Wallace, in his admirable dis- 

 cussion, considers the previously suggested actions to be adequate 

 for a glacial period. For my part, here for the first time I see an 

 action seeming certainly able to produce an appreciable effect. And 

 it has yet to pass through its ordeal of criticism. 



1 These calculations use Dulong and Petit's formula for radiation, and the common 

 assumption that there is a "Temperature of Space" of 150 C below freezing. 

 Neither is at all satisfactory, but the results probably serve fairly for comparisons. 



