ts_, Vol. XVI. 1878.

[45] In a letter to _Nature_ of October 30th, 1879, the Rev. O. Fisher
calls attention to a result arrived at by Pouillet, that the temperature
which the surface of the ground would assume if the sun were extinguished
would be -128deg F. instead of -239deg F. If this corrected amount were
used in our calculations, the January temperature of England during the
glacial epoch would come out 17deg F., and this Mr. Fisher thinks not low
enough to cause any extreme difference from the present climate. In this
opinion, however, I cannot agree with him. On the contrary, it would, I
think, be a relief to the theory were the amounts of decrease of
temperature in winter and increase in summer rendered more moderate, since
according to the usual calculation (which I have adopted) the differences
are unnecessarily great. I cannot therefore think that this modification of
the temperatures, should it be ultimately proved to be correct (which is
altogether denied by Dr. Croll), would be any serious objection to the
adoption of Dr. Croll's theory of the Astronomical and Physical causes of
the Glacial Epoch.

The reason of the theoretical increase of summer heat being greater than
the decrease of winter cold is because we are now nearest the sun in winter
and farthest in summer, whereas we calculate the temperatures of the
glacial epoch for the phase of precession when the _aphelion_ was in
winter. A large part of the increase of temperature would no doubt be used
up in melting ice and evaporating water, so that there would be a much less
increase of sensible heat; while only a portion of the theoretical lowering
of temperature in winter would be actually produced owing to equalising
effect of winds and currents, and the storing up of heat by the earth and
ocean.

[46] Dr. Croll says this "is one of the most widespread and fundamental
errors within the whole range of geological climatology." The temperature
of the snow itself is, he says, one of the main factors. (_Climate and
Cosmology_, p. 85.) But surely the temperature of the snow must depend on
the temperature of the air through which it falls.

[47] In an account of Prof. Nordenskjoeld's recent expedition round the
northern coast of Asia, given in _Nature_, November 20th, 1879, we have the
following passage, fully supporting the statement in the text. "Along the
whole coast, from the White Sea to Behring's Straits, no glacier was seen.
During autumn the Sib