CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 



125 



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. 



Difference of Temperature 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. Our zero is thirty-two degrees below the freezing 

 point of water, or, in the centigrade thermometer, the freezmg 

 point itself, both of which are equally misleading when applied 

 to cosmical problems. If we say that the mean temperature of 

 a place is 50° F. or 10° C, these figures tell us nothing of how 

 much the sun warms that place, because if the sun were with- 

 drawn the temperature 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., we may be sure that 

 even this intense cold was not near the possible minimum 

 temperature. 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 — 289° 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 by the sun to an amount 

 measured by 50 -f 239 = 289° F., which is hence termed its 

 absolute temperature. Now during the time of the glacial 

 epoch the greatest distance of the sun in winter was 97^ 

 millions of miles, whereas it is now, in winter, only 91 millions 

 of miles. But the quantity of heat received from the sun is 

 inversely as the square of the distance, so that it would then be 

 in the proportion of 8,281 to 9,506 now, or nearly one eighth 

 less than its present amount. The mean temperature of Eng- 

 land in January is about 39° F., which equals 278° F. of absolute 

 temperature. But the above named fraction of 278° is 36°, 

 representing the amount which must be deducted to obtain the 

 January temperature during the glacial epoch, which will there- 

 fore be 3° F. Our actual temperature at that time might, 



