82. J. Croll—Kzamination of Wallace's Modification of the 
that Mr. Wallace has advanced in explanation of geological 
climate I fully agree, but I am, nevertheless, wholly unable to 
perceive that any of his arguments or considerations do in 
reality materially affect the theory advocated in ‘Climate and 
Time.’ is I hope presently to show. 
Before proceeding, however, to examine in detail Mr. Wal- 
lace’s modifications of the theory, it may be as well to consider 
one or two minor points on which I differ from him, as this will 
save the necessity of referring to them when we come to dis- 
cuss his main argument. 
Effect of Winter Solstice in Aphelion.—At page 126 (‘Island 
Life’) he says :—“‘ We may therefore say generally, that during 
our northern winter, at the time of the Glacial epoch, the north- 
ern hemisphere was receiving so much less heat from the sun 
as to lower its surface-temperature on an average about 35° F., 
while during the height of summer of the same period it would 
be receiving so much more heat as would suffice to raise its 
mean temperature about 60° F. above what it is now.” Ina 
foot-note he adds that “the reason of the increase of summer 
heat being 60° while the decrease of winter cold is only 35°, is 
because our summer is now below and our winter above the 
average.” 
There is surely a confusion of ideas here. It is of course 
true that, as our summer at present occurs in aphelion and our 
winter in perihelion, the temperature of the former is below 
and that of the latter above the average; but this can afford 
no grounds for the result Mr. Wallace attributes to it unless it 
be assumed (for which there are no astronomical grounds) that 
“our summer is 25° further below the average than our winter is 
above it. 
On the Storage of Cold.—In a section on the Effects of Snow 
on Climate, Mr. Wallace points out the different effects pro- 
uced by water falling as a liquid in the form of rain and as a 
solid in the form of snow. The rain, however much of it may 
fall, rans off rapidly, he states, without producing any perma- 
nent effect on temperature. But if snow falls, it lies where it 
fell, and becomes compacted into a mass which keeps the earth 
below and the air above, at or near the freezing-point hen 
the snow becomes perpetual, as on the summits of high moun- 
tains, permanent cold is the result; and however strong the 
sun’s rays may be, the temperature of both the air and the 
earth cannot possibly rise much above the freezing-point. 
“This,” be says, ‘is illustrated by the often-quoted fact that 
at 80° N. lat. Captain Scoresby had the pitch melted on the 
one side of bis ship by the heat of the sun, while water was 
freezing on the other side owing to the coldness of the air.” 
Doubtless this is perfectly correct; but on page 502 he states 
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