ANCESTRAL MAN. 1 3 



consequence of the precession of the equinoxes, which effects a 

 revolution of the equinoctial point upon the ecliptic once in 

 21,000 years, it would occur many times that our northern winter 

 would happen in aphelion, or when the earth is farthest from the 

 sun. It is true that, in accordance with Kepler's law,* the 

 northern and southern hemispheres receive equal quantities of 

 sun-heat in summer, and equal, though different, quantities of 

 sun-heat in winter, whatever may be the eccentricity of the earth's 

 orbit ; because greater proximity of the sun is compensated by 

 swifter motion of the earth. But quantity of heat is not the same 

 thing as intensity, as may be understood by considering that from 

 May 10th till August 3rd, the quantity of heat received at the 

 north pole from the sun, in consequence of his remaining 

 continually above the horizon, is actually greater than the quantity 

 received, during the same eighty-five days, at the equator. The 

 difference between quantity and intensity is also shown by the 

 familiar fact that however much heat ice may receive, its tempera- 

 ture is not raised above 32 F. 



Moreover, in the case we are supposing, with a maximum 

 terrestrial eccentricity and the northern winter in aphelion, the 

 total sun-heat then received would be distributed over a greater 

 lapse of time. 



Our present winter is nearly 8 days shorter than our summer ; 

 but at the superior limit of eccentricity the winters of aphelion 

 would be 36 days longer than the summers. The result of this 

 lengthened hybernation would be a greater accumulation of 

 northern snow which a short summer, even though a hot one, 

 would be quite unable to melt. For the solar rays that fall on 

 snow or ice are to a great extent reflected back into space. It 

 has been found that where the land is covered with snow, even 

 the summer temperature seldom rises above the freezing point, as 

 the air cannot be warmed by the sun as quickly as it is cooled by 

 radiation to ice, while the heat so parted with becomes latent and 

 disappears in the mechanical work of liquefaction. And it has 

 been observed at the antarctic continent, where a minor glaciation 

 is at present in progress, that the summer weather consists of a 

 succession of snow-storms and fogs. 



Other effects would follow. The difference of temperature 

 between the equatorial and the polar regions would be greater 

 on the northern than on the southern hemisphere ;f and as a 

 consequence, the aerial currents of the northern hemisphere would 

 be stronger than those of the southern, because strength of wind 



* The radii vectores of the planets describe equal areas in equal times. 



t At present the mean annual temperature at the equator is 80.1° F.; at 60° North 

 Latitude it is 29.3 ° F.; and at 60 ° South Latitude it is 35.3 o F. Thus the difference 

 referred to is, at present, upwards of one-seventh greater in the northern than in the 

 southern hemisphere. In the case supposed the difference would be more considerable. 



