Mathematical Theories of the Earth. — Woodward. 277 
Poisson wrote much on the whole subject of terrestrial temperatures, 
and carefully considered most of the troublesome details which lay 
between his theory and its application. While he admitted the neb- 
ular hypothesis and an initial fluid state of the earth, he rejected the 
notion that the observed increase of underground temperature is due 
to a primitive store of heat. If the earth was originally fluid by reason 
of its heat, a supposition which Poisson regarded (^uite gratuitous, he 
conceived that it must cool and consolidate from the center outwards ; 
so that according to this view the crust of our planet arrived at a con- 
dition of stability only after the supply of heat had been exhausted. 
But Poisson was not at a loss to account for the observed temperature 
gradient in the earth's crust. Always fertile in hypothesis, he ad- 
vanced the idea that there exist, by reason of interstellar radiations, 
great variations in the temperature of space, some vast regions being 
comparatively cool and others intensely hot, and the present store of 
terrestrial heat was acquired by a journey of the solar system through 
one of the hotter regions. "Such is," he says, "in my opinion, the 
true cause of the augmentation of temperature which occurs as we 
descend below the surface of the globe." This hypothesis was the 
result of Poisson's mature reflection, and as such is well worthy of 
attention. The notion that there exist hot foci in space was advanced 
also in another form in 1852 by Rankine, in his interesting speculation 
on the reconcentration of energy. But whatever we may think of the 
hypothesis as a whole, it does not appear to be adequate to the case 
of the earth unless we suppose the epoch of transit through the hot 
region exceedingly remote and the temperature of that region exceed- 
ingly high. The continuity of geological and palwontological phenom- 
ena is much better satisfied by the Leibnitzian view of an earth long 
subject to comparatively constant surface conditions but still active 
with the energy of its primitive heat. 
Notwithstanding the indefatigable and admirable labors of Fourier 
and Poisson in this field, it must be admitted that they accomplished 
little more than the preparation of the machinery with which their 
successors have sought and are still seeking to reap the harvest. The 
difficulties which lay in their way were not mathematical but physi- 
cal. Had they been able to make out the true conditions of the 
earth's store of heat, they would undoubtedly have reached a high 
grade of perfection in the treatment of the problem. The theory as 
they left it was much in advance of observation, and the labors of their 
successors have therefore necessarily been directed largely towards 
the determination of the thermal properties of the earth's crust and 
mass. 
Of those who in the present generation have contributed to our 
knowledge and stimulated the investigation of this subject, it is hard- 
ly necessary to say that we owe most to Sir William Thomson. He 
has made the question of terrestrial temperatures highly attractive and 
instructive to astronomers and mathematicians, and not less warmly 
