432 Climate of the Glacial Period. (October, 
being set up? If currents in the air are caused by the 
unequal heating of different portions of it, why should not 
currents in the ocean be in like mannerset in motion? Mr. 
Carpenter contends, and has illustrated by experiment, 
that they are; and if he be correct, instead of the Gulf 
Stream being lessened by the increase of ice in the north, it 
would be greatly augmented; and I have already shown 
that there is evidence of its existence and influence in the 
glacial period. 
Another cause that Mr. Croll thinks might be a means of 
increasing the vicissitudes of temperature produced by the 
eccentricity of the orbit, is a change in the obliquity of the 
ecliptic. Accepting the conclusions of some eminent 
astronomers that the obliquity of the ecliptic can only vary 
to a small extent, he yet considers that this small amount 
would cause a great change of temperature; that when the 
obliquity was at its maximum, or, according to Laplace, 
24°50’ 34”, there would be an increase of temperature at the 
poles equal to 14° or 15° if they were not covered with ice, 
but if they were, then the total quantity of ice melted at 
the poles would be one-eighteenth more than at the present.* 
On the contrary, when the obliquity was at its minimum, 
there would be a decrease of temperature at the poles and 
an increase of the ice covering them. ‘This struck me 
when I first read it as a most extraordinary conclusion, and 
I considered it must have been the result of an inadvertence, 
as it appeared obvious that the effe¢t would be just the re- 
verse of that stated. But I find that Mr. Geikie, in his 
recent work, follows Mr. Croll in this as in other matters, 
and states that ‘if the obliquity of the ecliptic reached a 
minimum during our glacial epoch, as indeed it must have 
done more than once, the effect of the great eccentricity and 
diminished obliquity combined would be to intensify the 
glaciation of our hemisphere.” t 
As, in the former argument, I have had occasion to show 
that the radiation of heat by the earth during the day had 
been neglected, so in this calculation the all-important fact 
has been overlooked, that if the obliquity of the ecliptic be 
increased, the arctic circle will be enlarged and a greater 
area of the earth’s surface brought within the influence of 
the long arctic night. A diminished obliquity, on the con- 
trary, would lesson the difference in the temperate zones 
between the length of the night and day, and in so far 
moderate the extremes of cold and heat in winter and 
4 Philusophical Magazine, vol. xxxiii, p. 436. 
+ Great Ice Age, p. 147, 
