Travees. — 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 niicleus 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 

 centr§, 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 witjiin 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 cause, 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 3^our 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 carefull}- 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 



