with
relative nearness to the sun, during a period of 13,000 years. But there
is also a still slower change in the direction of the axis major of the
Earth's orbit; from which it results that the alternation we have
described is completed in about 21,000 years. That is to say, if at a
given time the Earth is nearest to the sun at our mid-summer, and
furthest from the sun at our mid-winter; then, in 10,500 years
afterwards, it will be furthest from the sun at our mid-summer, and
nearest at our mid-winter. Now the difference between the distances from
the sun at the two extremes of this alternation, amounts to
one-thirtieth; and hence, the difference between the quantities of heat
received from the sun on a summer's day under these opposite conditions
amounts to one-fifteenth. Estimating this, not with reference to the
zero of our thermometers, but with reference to the temperature of the
celestial spaces, Sir John Herschel calculates "23 deg. Fahrenheit, as the
least variation of temperature under such circumstances which can
reasonably be attributed to the actual variation of the sun's distance."
Thus, then, each hemisphere has at a certain epoch, a short summer of
extreme heat, followed by a long and very cold winter. Through the slow
change in the direction of the Earth's axis, these extremes are
gradually mitigated. And at the end of 10,500 years, there is reached
the opposite state--a long and moderate summer, with a short and mild
winter. At present, in consequence of the predominance of sea in the
southern hemisphere, the extremes to which its astronomical conditions
subject it, are much ameliorated; while the great proportion of land in
the northern hemisphere, tends to exaggerate such contrast as now
exists in it between winter and summer: whence it results that the
climates of the two hemispheres are not widely unlike. But 10,000 years
hence, the northern hemisphere will undergo annual variations of
temperature far more marked than now.

In the last edition of his _Outlines of Astronomy_, Sir John Herschel
recognizes this as an element in geological processes; regarding it as
possibly a part-cause of those climatic changes indicated by the records
of the Earth's past. That it has had much to do with those larger
changes of climate of which we have evidence, seems unlikely, since
there is reason to think that these have been far slower and more
lasting; but that it must have entailed a rhythmical exaggeration a