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 by 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 freezing 
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 — 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 by the sun to an amount 
measured by 50 + 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 971- 
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, 
