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Rev. E. Hill—Eccentricity and Glacial Epochs. Ee 
Before discussing these arguments in detail, it will be well to 
clear our minds from a misconception. Dr. Croll ascribes much 
importance to the greater length of the winter in periods when the 
eccentricity is large. This length, when the eccentricity had reached 
its highest value, might exceed its present limits by as much as 
forty-four days. Forty-four days more winter sounds enormous at 
first. Six weeks more of the severest cold, six weeks more skating 
and snow, would indeed be a change in climate. But this will not 
be the change in any temperate region. Dr. Croll is using the 
technical astronomical definition of winter, the time between the 
autumn and the spring equinoxes, the period during which the day 
is less than 12 hours long. ‘This period would, no doubt, be 
lengthened by forty-four days. But practically in England there 
are hardly more than two months in which snow can resist the sun. 
We seldom get skating except in December or January. Six weeks 
additional to half the year gives a fortnight additional to two months. 
If two months be a fair measure of the magnitude of our present 
winter, the increase during maximum eccentricity would be a fort- 
night, which is somewhat less alarming. Further, not only practi- 
cally, but theoretically, this technical definition of winter is a bad one. 
The only important period which it represents is the period of day- 
light at the Pole. If we want to divide the year into a cold half 
and a warm half, the equinoxes are not the proper points of division. 
For they do not divide the year into halves of equal length, and the 
amount of heat received in them is neither a mean nor a fixed 
amount. Although their lengths are the same, all years alike, no 
matter what be eccentricity or obliquity, the amount of heat received 
during them is not the same, for the distance of the sun differs with 
differing eccentricities. The most natural way of fixing the division 
for any place would be to calculate the mean amount of heat received 
per day on the average of the whole year. Then let all the days 
during which the heat received from the sun exceeded this average 
diurnal amount be called Summer, and the period during which the 
daily heat fell short of this average be Winter. This would give 
a length for Summer, subject to variation by change of eccentricity, 
to an extent greatest at the pole, diminishing towards the equator, 
and thus would more nearly represent the true state of the case. 
There is not much importance in this difference of definition, 
Indeed, in dealing with our subject it is better to use neither. The 
simplest and clearest division is that into halves of equal length. 
Dr. Croll divides his year into two parts, in which the amounts of 
heat received are equal, but the times unequal. We shall find it 
more useful to take two equal times in which the amounts of heat 
received will be different. The six months nearest to the Winter 
Solstice we will call Winter, and those nearest to the Summer 
Solstice, Summer. Our summer will be, roughly, from April to 
September; our winter, October to March. All the hottest days will 
be in one half, all the coldest in the other. The line between sum- 
mer and winter is then so drawn as to divide the heat received most 
unequally. We will neglect for the present (as Dr. Croll does) the 
