1875.] The History of our Earth. 315 
eccentricity of the earth’s orbit. In the northern hemi- 
sphere, at present, winter occurs when the earth is nearest 
to the sun. As at that period she travels in her orbit with 
the greatest speed, it results that our winter half-year— 
z.e., the interval between the autumnal and the vernal 
equinox—is nearly eight days shorter than the summer 
half-year. In the southern hemisphere the conditions are, 
of course, exactly reversed. In virtue of the precession of 
the equinoxes, however, times have been—and will be again 
—when the northern hemisphere will have its winter when 
the earth is in aphelion, and its summer when she is in 
perihelion, and when it, in turn, will have a longer winter 
and ashorter summer. But the orbit of the earth is also 
undergoing a regular series of changes. Its ‘‘ eccentricity is 
at present diminishing, and will continue to do so during 
23,980 years from the year 1800 A.D.” After this time it 
will again gradually increase. ‘The earth’s distance from 
the sun, when in perihelion, is at present 89,000,000 miles. 
But when the eccentricity of her orbit is at its superior limit 
it will have fallen to 84,000,000, whilst her distance when in 
aphelion will be no less than 98,000,000. Hence, when the 
eccentricity is greatest, that hemisphere which has its winter 
solstice in aphelian will have its winter longer than its 
summer, not by eight, but by thirty-six days. The direct 
heat of the sun would be one-fifth less during that season 
than it is at present. Hence there would be greater facilities 
for the accumulation of snow and ice, and less opportunity 
for their disappearance during the summer. Now there is 
perhaps no effect which reacts upon and intensifies its cause 
so decidedly as do snow and ice. ‘These substances, when 
once accumulated, lower the temperature in various ways. 
No matter what the intensity of the sun’s rays may be, the 
temperature of ice and snow can never rise above 32° F. 
fin Greenland,’ as Mr. Croll reminds, us; -“‘a country 
covered with snow and ice, the pitch has been seen to melt 
on the side of a ship exposed to the direct rays of the sun, 
while at the same time the surrounding air was far below 
the freezing-point.” The atmosphere is chilled by contact 
with the snow-covered ground, and is not warmed by the 
radiation from the sun. “Again, the greater part of the rays 
which fall on snow and ice are refleéted back into space and 
wasted. Those which are not reflected away cannot raise 
the temperature, since they are consumed in the mechanical 
work of melting the ice. Further, snow and ice chill the 
air, and condense its vapours in the form of thick fogs, 
and thus effectually screen themselves against the rays “of 
the sun. 
