i877-] 
Probable Origin and Age of the Sun . 
325 
It may now be admitted as settled that if the earth be 
perfectly rigid the climate of our globe could never possibly 
have been affedled by any change in the axis of rotation. 
But it is maintained that if the earth can yield as a whole, 
so as to adapt its form to a new axis of rotation, the effects 
may be cumulative, and that a displacement of the Pole as 
much as io° or 15' is possible.*' 
But then if the earth be able to adapt its form to a 
change in the axis of rotation , there is no reason why it may 
not be able to adapt its form to a change in the rate of rotation , 
and, if so, the flattening at the Poles and the bulging at the 
Equator would diminish as the rate of rotation diminished, 
even supposing there were no denudation going on. 
Argument from the Secular Cooling of the Earth.— The 
earth, like the sun, is a body in the process of cooling, and 
it is evident that if we go back sufficiently far we shall reach 
a period when it was in a molten condition. Calculating by 
means of Fourier’s mathematical theory of the conductivity 
of heat, Sir William Thomson has endeavoured to determine 
how many years must have elapsed since solidification of the 
earth’s crust may have taken place. This argument is un- 
doubtedly the most reliable of the three. Nevertheless, 
the data on the subject are yet very imperfect, so that no 
definite and trustworthy result can be arrived at by this 
means as to the aCtual age of the earth. In faCt this is ob- 
vious from the very wide limits assigned by him within which 
solidification probably took place. “ We must,” quoting 
Sir William’s own words on the subject, “ allow very wide 
limits on such an estimate as I have attempted to make ; 
but I think we may, with much probability, say that the 
consolidation cannot have taken place less than 20,000,000 
years ago, or we should have more underground heat than 
* A displacement of the Pole of less than 15 0 or 20° would be of very little 
service in accounting for the warm climate of Greenland during the Miocene 
and other periods. But a displacement to that extent, even supposing we 
admit the earth to be yielding, demands a condition of things which few geol- 
ogists would be willing to grant. When it becomes generally recognised to 
what an enormous extent the temperature of the Ardtic regions is dependent 
upon ocean currents, the difficulties in understanding how those regions have 
once enjoyed a temperate climate will disappear. Were the ice removed from 
Greenland that region would at present enjoy a warm summer, suitable for 
plant and animal life. It is the presence of ice rather than a positive defi- 
ciency of heat that makes Greenland so cold and barren (see Climate and 
Time, Chap. IV.). An increase in the quantity of heat conveyed by ocean- 
currents, merely sufficient to prevent the accumulation of ice, would com- 
pletely transform the climate of the Ardtic lands. And such an increase 
would take place during an Inter-glacial period when the eccentricity of the 
earth’s orbit was at a high value and the winter solstice in perihelion, 
