106 G. F. Becker — Certain Astronomical 



of the general truth of this result. For an increase in the 

 length of the winter diminishes all heat-rates or temperatures 

 in the same proportion ; and since the rates are highest in the 

 tropics, the greatest decrease must also take place in the 

 torrid belt. The indications of the diagram are that the tem- 

 perature of January in the eccentric period would be greatest 

 at the tropic of Capricorn, and that it would there be no 

 warmer than it now is in July at 45° of north latitude. It 

 follows that the evaporation would be very small and that the 

 snowfall in high northern latitudes would be small. The heat 

 gradient during the eccentric winter is also less steep than it is 

 possible to make it by any other combination of conditions, and 

 therefore the winter of this period in the northern hemisphere 

 would be the calmest, dryest and coldest possible. 



The summer of the eccentric period in the hemisphere 

 of rigorous climate will be the hottest possible ; nearly 20° 

 F. hotter, it would seem, than that of the present time in 

 temperate latitudes. The evaporation would of course be 

 immense. The heat gradient toward the pole is also consider- 

 ably greater than it now is, or than it would be at the time of 

 zero eccentricity. Hence the summer would be wet as well as 

 hot. It seems to me, then, that the period of greatest eccen- 

 tricity would be most unfavorable to glaciation, the snowfall 

 being the smallest, and the summer rainfall the largest which 

 can occur with the present obliquity. It seems much less 

 favorable than the period of zero eccentricity when the winter 

 cold is great enough to preclude much rain in the higher por- 

 tion of the temperate zone, while the temperature in the 

 tropics is great enough to produce active evaporation. It 

 would be manifestly absurd to suppose equality of seasons 

 sufficient to produce an ice age ; but I am forced to the con- 

 clusion that, so far as eccentricity is concerned in the matter at 

 all, the smaller the eccentricity the more favorable are the con- 

 ditions for glaciation. 



Thus far only a slight reference has been made to the varia- 

 tion of the obliquity. Its influence on glaciation has been 

 considered by various authors, and Dr. Croll attributes to a 

 greater obliquity a considerable influence on the temperature 

 of the arctic circle. The greatest possible obliquity of the 

 apparent ecliptic is believed to be 24° 36', and I have adopted 

 this value for computation.* The heat-rates are recorded in 

 the table, and the corresponding curves are shown in figure 3. 



At first sight the winter curve of the period of greatest 

 obliquity seems practically undistinguishable from that for the 

 present obliquity, the eccentricities being in both cases zero. 



* Stock well gives as the greatest obliquity 24° 35' 57 "53 for the values of the 

 masses of the planets which he adopts. Smiths. Cout., vol. xviii, 1873, p. 175. 



