THE ICE AGE. — CLIMATE AND TIME. 
241 
can move upon such a surface only in consequence of pressure 
acting from the interior ; that is, the ice sheet must thicken from 
the edge inwards and then flow upon itself. Mr. Croll estimates 
that a slope of one degree, continued for 1,400 miles, will give 
24 miles as the thickness of ice at the pole. No such thickness 
as this exists, hut it is known to be very considerable. Such 
land-ice moves slowly, hut it exerts enormous pressure. A 
glacier of 1,000 feet in thickness has a pressure upon its bed 
equal to about 25 tons on every square foot ; consequently, as it 
extends itself, it carries with it all that it gathers in its course 
— the results of its own irresistible power. The inference to be 
drawn from all the facts referred to, is that there certainly was 
a time when all this country was under a considerable thick- 
ness of ice — the result of a very low temperature extending over 
an exceedingly large section of the globe. 
The questions which naturally arise from the contempla- 
tion of the phenomena connected with the changes of climate 
in the earlier history of our globe are — At what period of geo- 
logical time did the last glacial epoch occur ? for how long did 
it last ? and when did it terminate ? 
We must endeavour briefly to give the answers to those 
questions which are afforded by the investigations of science. 
At one period there prevailed an idea that the subterranean 
heat of our globe exerted a considerable influence upon the 
condition of its surface temperature. Sir William Thomson 
appears to have proved that the general climate of our globe 
eould not have been sensibly affected by internal heat, at any time, 
more than 10,000 years after the commencement of the solidi- 
fication of the surface, and he states that the present influence 
of internal heat on the surface temperature, amounts to about 
only 1-7 5th of a degree. Mr. Croll, in his “ Climate and Time,” 
says, “ Not only is the theory of internal heat now generally 
abandoned, but it is admitted that we have no good geological 
evidence that climate was much hotter during Palaeozoic ages 
than now, as some have somewhat hastily asserted, and much 
less that it has been becoming uniformly colder.” 
Certain it is that the temperature of the Earth is re- 
gulated by radiation of heat from the Sun. Supposing this 
to be a constant quantity, as we have every reason to do, how 
is it that we have evidences of great climatic changes? We 
know that an arctic condition of climate prevailed in our 
island, and that most of the temperate regions, down to com- 
paratively low latitudes, were buried under ice, and that at 
another period Greenland and the arctic regions were not only 
free from ice, but were covered with a rich and luxuriant 
vegetation. 
To explain these great changes, Poisson and others following 
VOL. XIV. — NO. LVI. R 
