in Geological Climatology. 263 
The primary cause of all these physical agencies being set 
in operation is a high state of eccentricity of the earth's 
orbit ; and with a continuance of that state a glacial epoch 
becomes inevitable. 
The Explanation begins icith Winter. — Mr. Hill asks why I 
always begin in my explanation with the aphelion winter 
rather than with the perihelion summer. The reason is that 
the character of the summer is determined by that of the 
winter, and not the winter by that of the summer. It is 
true that to a certain extent the influence is mutual ; but the 
effect of the summer on the winter is trifling in comparison 
with that of the winter on the summer. To begin our expla- 
nation with the summer would be like beginning at the end 
of a story and telling it backward. 
M. Woeikof on the Cause of Glaciation. — In an article by 
A. "VToeikof on " Glaciers and Glacial Periods in their rela- 
tions to Climate" (' Nature,' March 2nd, 1882), it is main- 
tained that the chief cause which leads to the formation of 
snow, and consequently to a glacial condition, is a low surface- 
temperature of the sea surrounding or adjoining the land. 
When the surface-temperature of the water much exceeds the 
freezing-point, the vapour, he says, evaporated from the sea 
and condensed on the land will be rain and not snow ; but 
when the temperature of the water is near the freezing-point, 
snow will be the result. A diminution, for example, in the 
heat brought by the Gulf-stream that would very greatly 
lower the surface-temperature of the sea surrounding Great 
Britain would, he says, bring about a heavy snowfall and 
lead to permanent snow and ice. Again, he maintains, " As 
there is no reason to suppose that the surface-temperature of 
the sea would be lower during winter in aphelion and high 
eccentricity, it follows that there will not be more snow than 
now in countries where rain is the rule, even in winter, all 
other things equal." 
There is surely a fallacy lurking under this theory of M. 
Woeikof. Snow instead of rain is not, as he supposes, owing 
to the low temperature of the water from which the vapour 
is derived, but to the low temperature of the air where the 
vapour is precipitated. Of course, when the surface of the 
sea is near the freezing-point, the air over the sea and the 
adjoining land is usually also not far from the freezing-point, 
and consequently the precipitation is more likely to be snow 
than rain. If the air be cold, as it generally is over a snow- 
and ice-covered country, a high temperature of the adjoining 
seas, were this possible, would greatly increase the snowfall, 
