1874.) Climate of the Glacial Period. 445 
great mass of ice—some thousands of feet thick—that 
moved down southwards over the northern parts of America, 
Europe, and Asia, could not have been the production of a 
single winter. It is possible that this and some other 
geological speculations of the author have prevented many 
from taking a favourable view of his theory, and it is of 
importance to discuss what would be the real effect of a 
greater obliquity of the ecliptic. 
We are able to approach this question provided with 
data derived from the effeCis of the present inclination of 
the axis of the earth to the plane of its orbit. To it is due 
the varying length of the day throughout the year in the 
temperate and arctic zones, and the consequent production 
of theseasons. If the axis, as in Jupiter, were perpendicular 
to the plane of the orbit, night and day throughout the 
world would be equal. Every day there would be twelve 
hours’ light and twelve hours’ darkness. Each place would 
have but one season, and eternal spring would reign around 
the arctic circle. Under such circumstances the piling up 
of snow, or even its production at the sea-level, would be 
impossible, excepting perhaps in the immediate neighbour- 
hood of the poles, where the rays of the sun would have 
but little heating power from its small altitude. 
Our summer and winter are therefore due to the present 
obliquity of the ecliptic, and so also is it that now around 
the poles some lands are being glaciated, for excepting 
for that obliquity snow and ice could not accumulate, 
excepting on mountain chains. The obliquity of the 
ecliptic does not affect the mean amount of heat received 
at any one point from the sun, but it causes the heat 
and the cold to predominate at different seasons of the 
year. Near the poles there are six months’ night and 
six months’ day, but the absolute amount of heat that 
arrives from the sun is the same as if there were twelve 
hours’ light and twelve hours’ darkness every day. The 
cause of perpetual ice and snow is not, as I have already 
shown, the cooling of the air by the melting snow in 
summer, nor the formation of clouds shutting off the rays 
of the sun. It is, I believe, in consequence of the reflection 
into space of many of the rays of light and heat that fall 
on a snow-covered surface, and any cause that tends to 
increase the amount of snow or to extend the snow-covered 
area will tend to chill the climate of such parts by occa- 
sioning more of the rays of the sun to be deflected and lost. 
Therefore a long hot summer and a long cold winter are 
more likely to favour the accumulation of perpetual snow 
VOL. IV. (N.S.) 3k 
