of the Polar Regions. 67 
cene epoch is too nearly approximate to our own to allow of 
our attributing to it, with any probability, the difference of 
temperature now indicated. 
It seems to us to follow, from the preceding considerations, 
that it is from the study of phenomena of another kind that 
we must obtain the solution of the problem that we seek. Let us 
first examine, in the series of cosmical poenemene, the question. 
of the changes which may have taken place in the position of the 
earth relatively tothe sun. From the point of view which now 
engages our attention, a great importance has been quite re- 
cently attached to the periodical modifications of the eccen- 
tricity of the ellipse which is annually traversed by the earth. 
The form of this ellipse is modified within certain limits in 
the course of ages. At present it approaches a circle ; in .23,900 
years its eccentricity will have attained its minimum ; then 
the orbit will again tend to acquire a more elongated form. 
The mean distance from the earth to the sun is 91,400,000 
miles; when the eccentricity of the ellipse is at its maximum 
it has +, of this length, when it is at its minimum ;};; in 
the former case the earth departs from the sun 14,500,000 
miles more than in the second case. At present the linear 
value of the eccentricity is three millions of miles. It must 
also be observed that, at present, the earth is nearest to the 
sun (at the perihelion) during the winter of the northern 
hemisphere, whilst in the summer it is furthest from it (at the 
aphelion). Now the relative position.of the line of apsides 
and of that of the solstices is likewise subjected to a movement 
of revolution which is accomplished in 21,000 years. In 
about 10,000 years the summer of the northern hemisphere 
will fall at the period when the earth is nearest to the sun, 
and its winter at that when it is most distant. Of course the 
contrary will take place in the southern hemisphere. 
It is assumed that during the periods when the eccentricity 
of the orbit of the earth approaches its maximum, when the 
rihelion coincides with the winter solstice, the northern 
emisphere must enjoy a shorter and warmer winter, whilst 
the summer is longer and cooler. During this time the oppo- 
site is the case in the southern hemisphere. Its winter is 
: _ longer and colder, its summer hotter and shorter, because the 
winter of this hemisphere corresponds with the greatest dis- 
tance from the sun. Mr. Croll supposes that during this long 
and cold winter so great a quantity of ice must have been 
9 accumulated, that the summer following it (which, although 
hot, was short) had not the power to melt it entirely, and that 
it is at this epoch that we must place the glacial period. In 
the northern hemisphere, on the contrary,ethere would have 
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