104 G. F. Becker — Certain Astronomical 



tain amount of sunshine in high latitudes will not seriously 

 diminish the accumulation of neve ; for a great part of the 

 winter snowfall has a temperature far below freezing ; and in 

 summer, water resulting from superficial melting will freeze 

 again as it percolates through subjacent snow until the entire 

 accumulation of the past winter is raised to the melting point. 

 Such a process is apparently essential to the formation of gla- 

 cier ice. While a portion of the direct sunshine is harmlessly 

 employed in converting snow to ice, another and very large 

 part will be reflected from the neve fields. Hence it seems to 

 me that the features of a summer climate in a glaciated hemis- 

 phere which are most favorable to ice accumulation are cool 

 tropics and a low temperature gradient toward the pole, even 

 if the direct sunshine in very high latitudes must be increased 

 to bring about a dry climate. 



It seems proper to begin a comparison between various 

 climates by considering the conditions in opposite hemispheres 

 at any time when the difference of seasons is considerable. 

 Since the total amount of heat received by a hemisphere 

 between equinoxes is wholly independent of the duration of 

 this interval, one hemisphere will then have long cold winters 

 with short hot summers, and the other will have long cool 

 summers and short warm winters. A comparison of curves 

 representing such conditions shows further, that in the genial 

 hemisphere in winter, the temperature gradient will be very 

 high and the zone of evaporation very hot, so that the weather 

 will be very wet as well as relatively warm. The summer in 

 the genial hemisphere on the contrary will be cool and not 

 particularly wet. The high winter temperature, and the cor- 

 responding brevity of the season in the genial hemisphere 

 would certainly preclude glaciation with the present mean 

 temperature of the globe. Hence in discussing the conditions 

 favorable to glaciation it is needless to consider those in 

 which the winter is shorter than the summer. This is a con- 

 clusion upon which, so far as I know, every one is agreed. 



A comparison may now be made between the present cli- 

 mates and that which would prevail in both hemispheres were 

 the eccentricity zero, the apparent obliquity of the ecliptic 

 maintaining its present value. The seasons would then be of 

 equal length, and the climate would be intermediate between 

 that of the present time in the two hemispheres. Five or six 

 thousand years ago the seasons were of equal length, and the 

 eccentricity is now and was then so small as to affect the 

 amount of heat received by an insignificant fraction. In fact 

 the earth now receives 1*00014 times as much heat as it would 

 do if the eccentricity were zero. The rates given in the table 



