Travers.—On the former Warmth in high Northern Latitudes. — 461 
slag, and a variety of other cases of the same kind, in order to show how 
untenable are the propositions of Poisson and his followers, that if the globe 
had ever passed from a liquid to a solid state, in consequence of the loss of 
heat by radiation, the cooling and consolidation of the nucleus would have 
begun at the earth’s centre. 
Now, assuming that the views of Poisson are untenable, and that the 
cooling of the globe commenced at the surface, and extended towards the 
centre, it is palpable that the loss of heat resulting from radiation must have 
been greatest in the earlier periods of the earth’s revolutions, and must have 
decreased in proportion as the solidified crust extended in depth. But, 
in our globe three different modes for the transmission of heat have been 
distinguished, the first being periodic and affecting the temperature of the 
crust according to the different positions of the sun and the seasons of the 
year; the second, also due to the sun, namely, that a portion of the heat of 
the sun which penetrates the crust in the equatorial regions moves through 
the crust towards the poles, where it escapes into the atmosphere; the third 
being derived from the secular cooling of the earth, and from the primitive 
heat still being disengaged from the surface. This latter has for many ages 
been very insignificant, owing chiefly to the fact that it is interrupted in its 
passage by an enormous thickness of sedimentary and other strata, which 
are very bad conductors of heat. Laplace has shown, by reference to 
astronomical observations taken in the time of Hipparchus, that within the 
last 2000 years no sensible contraction has taken place in the globe by 
cooling, but it must be borne in mind that such a period as 2000 years, vast 
as it may seem when taken in reference to ordinary historical events, is but 
as a fleeting second in the eras which have passed since our globe was con- 
densed into its original fluid mass, and that it is therefore highly improbable 
that in so comparatively short a time any appreciable change in the length 
of a day arising from such a causé, could have been ascertained. 
There is, however, a matter of very considerable importance in connec- 
tion with the present distribution of heat in the crust of the globe, to which 
I must call your attention, namely, the periodic changes of temperature 
occasioned on the earth’s surface by the sun’s position and by meteorological 
processes. Now, it has been ascertained, by carefully conducted experi- 
ments, that these changes are continued in the crust of the earth, though 
to an inconsiderable depth, but that they are, nevertheless, such as even 
now to exercise a very marked influence on vegetation, and, indeed, on life 
generally. The slow conducting power of the ground, which checks the 
loss of heat in winter, is favourable to the growth of deep-rooted trees. 
-* Points that lie at different depths on the same vertical line," says 
Humboldt, “ attain the maximum and minimum of imparted temperature 
