180 Ii.evie2cs — LyelVs Principles of Oeology. 



offer us a record. But there is often evidence of the climate having 

 been very different from what it is found to be in the same area now, 

 though the change has sometimes been from warmer to colder, and 

 sometimes vice versa. 



Our author discusses very fully the various astronomical causes 

 which may have influenced climate. Perhaps the most important 

 of these is the obliquity of the ecliptic, to which the phenomenon of 

 the seasons is due. " Whenever," says Sir Charles Lyell, " the 

 obliquity is greater than now, more of the arctic and antarctic 

 regions would be exposed to a long night in winter, .... and under 

 the opposite circumstances the reverse would take place." (p. 293.) 



There is, however, another condition to be considered. The earth 

 moves round the sun in an ellipse, having the sun in one of the foci, 

 so that the earth is much nearer to it at one part of the year than at 

 the other, and the excentricity of the orbit varies so that the dif- 

 ference between the distance when it is nearest and when it is 

 farthest is much greater at one time than another. "Still, as the 

 extreme amount of difference in the quantity of heat annually re- 

 ceived, owing to such change in the minor axis, can never, by possi- 

 bility, exceed the whole supply in a ratio of more than 1003 to 1000, 

 it may, he [Sir John Herschel] says, be neglected in our geological 

 •speculations." (p. 273.) 



Climate, however, depends not only upon the absolute amount of 

 heat received, but also upon the manner in which it is distributed 

 ■throughout the seasons ; and if " the extreme of possible obliquity 

 happened to combine with the maximum excentricity, and with 

 geographical circumstances of an abnormal character, like those 

 now prevailing in high latitudes, a greater intensity of cold would 

 be produced than could exist without such a combination." (p. 293). 

 It is therefore of great importance to consider which pole is turned 

 away from the sun in aphelion under such conditions as here sup- 

 posed, and astronomers show us that, owing to a combination of 

 movements, the earth's path is so modified at intervals of a little 

 more than 10,000 years, that the pole which was towards the sun 

 when the earth was nearest is now towards the sun when the earth 

 is farthest, and vice versa. It is a matter of calculation when this 

 combination of astronomical causes shall occur for either pole. Yet 

 without " geographical circumstances of an abnormal character," the 

 effect of such astronomical combinations would hardly be felt ; the 

 mean temperature would vary only within a very few degrees, and 

 the climate but little. But if we follow our author, we shall find 

 what a vast difference it makes in the climate of any region, whether 

 it has land all round to absorb and give out again the heat of a 

 tropical sun, or an ocean which will catch but little and slowly part 

 with what it has caught ; whether we have, collected round the pole, 

 land to gather winter snow and prove a vast refrigerator when 

 summer comes, or an ocean where little ice can be formed and little 

 snow be gathered. 



True to his Principles, Sir Charles Lyell examines what are the 

 conditions which prevail on the earth now in hemispheres and along 



