436 Mr. J. Croll on the Change in the Obliquity of the Ecliptic, 



period under consideration the total annual quantity of ice 

 melted at the poles would be -^ more than at present. 



The general effect which the change in the obliquity of the 

 ecliptic would have upon the climate of the polar regions when 

 combined with the effects resulting from the excentricity of the 

 earth's orbit would be this : — When the excentricity was at a 

 very high value, the hemisphere whose winter occurred in 

 aphelion (for physical reasons which have already been discussed 

 at considerable length*) would be under a condition of glacia- 

 tion, while the other hemisphere, having its winter in perihelion, 

 would be enjoying a warm and equable climate. When the ob- 

 liquity of the ecliptic was at a' maximum, and ^g- more heat 

 falling at the pole than at present, the effect would be to modify 

 to a great extent the rigour of the glaciation in the polar zone 

 of the hemisphere under a glacial condition, and, on the other 

 hand, to produce a more rapid melting of the ice on the other 

 hemisphere enjoying the equable climate. The effects of excen- 

 tricity and obliquity thus combined would probably completely 

 remove the polar ice-cap from off the latter hemisphere, and 

 forest-trees might then grow at the pole. Again, when the ob- 

 liquity was at its minimum condition and less heat reaching the 

 poles than at present, the glaciation of the former hemisphere 

 would be increased and the warmth of the latter diminished. 



One very remarkable effect which seems to result indirectly 

 from a variation of the obliquity, under certain conditions, is an. 

 influence on the level of the sea. As this probably may have had 

 something to do with those recent changes of sea-level which 

 the history of the submarine forests and raised beaches have 

 made us all so familiar with, it may be of interest to enter at 

 some length into this part of the subject. 



It appears almost certain that, at the time when the northern 

 winter solstice was in aphelion last, a rise of the sea on the 

 northern hemisphere to a considerable number of feet must have 

 taken place from the combined effect of excentricity and obliquity. 

 About 11,700 years ago the northern winter solstice was in the 

 aphelion. The excentricity at that time was *0187, being some- 

 what greater than it is now ; but the winters occurring in aphe- 

 lion instead of perihelion, as at present, they would on this account 

 be probably 10° or 15° colder than they are at the present day. 

 It is probable also, for reasons formerly statedf , that the Gulf- 

 stream at that time would be considerably less than at present. 

 This would tend to lower the temperature to a still greater ex- 

 tent. As snow must have fallen during winter instead of rain, 

 this no doubt would have produced a slight increase in the quan- 

 tity of ice on the northern hemisphere, had no other cause come 

 * Phil. Mag. for August 1864 and February 1867. t Ibid. 



