1872.] Jer reaerye 
been already given in the reference made to an Monian summer. 
This requires an explanation no less than .does the great winter 
which duly followed. 
Now there are certain astronomical facts which seem to point in 
just this direction. These may be simple stated as facts, indicating 
the probability of great summers and winters of the ages, and thus 
leading us to look upon Glacial periods as a part of the reeular 
course of nature. These facts, briefly stated, and they can be stated 
here only in the most general terms, are the following : — 
1. Variation in the obliquity of the earth’s axis to the plane of 
of the ecliptic. Such variations, there being a passage from one 
extreme to the other, and back again, are known to occur in given 
periods, though, so far as Iam aware, their exact duration has not 
been determined. 
2. Variation in the precession of the equinoxes, in connection 
with the advances of the perihelion. One of these has its revolu- 
tion in about one hundred and eleven thousand years, while the 
other completes its circuit in some twenty-five thousand eight hun- 
dred years. Their rapidity being unequal, they so revolve, each in 
its legitimate course, as to coincide in periods of about twenty-one 
thousand years’ length. 
3. Variation in the eccentricity of the enna: orbit. It is now 
well ascertained that the orbit of the earth undergoes changes, that 
it passes from the figure of a considerably flattened ellipse to a form 
almost circular —in cycles, each of which consists in round numbers 
of two hundred and thirty-four thousand years, thus causing the 
earth to vary greatly, at different times, in its distance from the sun. 
Let it be now remarked that each of the points just mentioned in- 
volves elements which occasion marked differences in the relation of 
the sun to the earth, and thus in the condition of the earth itself. 
Each one of them has been also referred to as sufficient to account 
for the cold of the Glacial period. No writer, however, so far as I 
am aware, has brought them altogether in a way to show just how 
and when, in the course of a large number of revolutions, the condi- 
tions in each favorable to extreme heat have all coincided, so those 
in each tending to the intensest cold have all coincided, in a manner 
to produce a great Monian summer, followed after a long interval 
by its alternate Monian winter. Such a combination, both of 
points already stated, and of others perhaps equally important, in- 
volves elements from which, when fully wrought out, conclusions 
