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24 COSMICAL ASPECTS OF GEOLOGY. [Boox I. 
when the earth was in the perihelion of its orbit, the earth would 
then be 14,368,200 miles nearer the sun in winter than in summer. — 
In this case the difference between winter and summer in our ~ 
latitudes would be almost annibilated. But as the winters in the 

one hemisphere correspond with the summers in the other, it follows — 
that while the one hemisphere would be enduring the greatest 
extremes of summer heat and winter cold, the other would be 
enjoying perpetual summer. i 
“Tt is quite true that whatever may be the eccentricity of the 
earth’s orbit, the two hemispheres must receive equal quantities of 
heat per annum; for proximity to the sun is exactly compensated 
by the effect of swifter motion. The total amount of heat received 
from the sun between the two equinoxes is therefore the same in both 
halves of the year, whatever the eccentricity of the earth’s orbit may 
be. For example, whatever extra heat the southern hemisphere may 
at present receive per day from the sun during its summer months, 
owing to greater proximity to the sun, is exactly compensated by a 
corresponding loss arising from the shortness of the season; and, on 
the other hand, whatever deficiency of heat we in the northern 
hemisphere may at present have per day during our summer half- 
year, in consequence of the earth’s distance from the sun, is also 
exactly compensated by a corresponding length of season. 
“Tt is well known, however, that those simple changes in the 
sun’s summer and winter distances would not alone produce a glacial 
epoch, and that physicists, confining their attention to the purely 
astronomical effects, were perfectly correct in affirming that no 
increase of eccentricity of the earth’s orbit could account for that — 
epoch. But the important fact was overlooked that, although the 
glacial epoch could not result directly from an increase of eccentricity, 
it might nevertheless do so indirectly from physical agents that 
were brought into operation as a result of an increase of eccentricity. 
The following is an outline of what these physical agents were, how 
they were brought into operation, and the way in which they may 
have led to the glacial epoch. 
“With the eccentricity at its superior limit and the winter cccur- 
ring in the aphelion, the earth would, as we have seen, be 8,736,420 
miles farther from the sun during that season than at present. The 
reduction in the amount of heat received from the sun, owing to his 
increased distance, would lower the midwinter temperature to an 
enormous extent. In temperate regions the greater portion of the — 
moisture of the air is at present precipitated in the form of rain, and 
the very small portion which falls as snow disappears in the course 
of a few weeks at most. But in the circumstances under considera- 
tion, the mean winter temperature would be lowered so much below 
the freezing-point that what now falls as rain during that season 
would then fall as snow. ‘This is not all; the winters would then 
not only be colder than now, but they would also be much longer. 
At present the winters are nearly eight days shorter than the 
