CHAP. VIII THE CAUSES OF GLACIAL EPOCHS 129 



years ago) the excentricity was very great, reaching a 

 maximum of three and a half times its present amount at 

 almost the remotest part of this period, at which time the 

 length of summer in one hemisphere and of winter in the 

 other would be nearly twenty-eight days in excess. Now, 

 during all this time, our position would change, as above 

 described (and as indicated on the diagram), every ten 

 thousand hve hundred years ; so that we should have 

 alternate periods of very long and cold winters with short 

 hot summers, and short mild winters with long cool 

 summers. In order to understand the important effects 

 which this would produce we must ascertain two things — 

 first, what actual difference of temperature would be caused 

 by varying distances of the sun, and, secondly, what are the 

 properties of snow and ice in regard to climate. 



Differences of Temperatio'e Caused hy Varying Distances of 

 the, Sun. — On this subject comparatively few persons have 

 correct ideas owing to the unscientific manner in which we 

 reckon heat by our thermometers. The zero of Fahren- 

 heit's thermometer is thirty-two degrees below the freezing 

 point of water, and that of the centigrade thermometer, 

 the freezing point itself, both of which are equally 

 misleading when applied to cosmical problems. If we say 

 that the mean temperature of a j^lace is 50° F., or 10° C, 

 these figures tell us nothing of how much the sun warms 

 that place, because if the sun were withdrawn the temper- 

 ature would fall far below either of the zero points. In 

 the last Arctic Expedition a temperature of— 74° F. was 

 registered, or 106° below the freezing point of water; and 

 as at the same time the earth, at a depth of two feet, was 

 only, — 13^ F. and the sea water -{-28° F., both influencing 

 the temperature of the air, we may be sure that even this 

 intense cold was not near the possible minimum tempera- 

 ture. By various calculations and experiments which 

 cannot be entered upon here, it has been determined that 

 the temperature of space, independent of solar (but not of 

 stellar) influence, is about —239^ F., and physicists almost 

 universally adopt this quantity in all estimates of cosmical 

 temperature. It follows, that if the mean temperature of 

 the earth's surface at any time is 50° F. it is really warmed 



