INTERNAL HEAT OP THE EABTH. 165 



That which has been learnt by an ingenious analytic cal- 

 culation, expressly perfected for this class of investigations*, 

 regarding the motion of heat in homogeneous metallic sphe- 

 roids, must be applied with much caution to the actual cha- 

 racter of our planet, considering our present imperfect know- 

 ledge of the substances of which the Earth is composed, the 

 difference in the capacity of heat and in the conducting power 

 of different superimposed masses, and the chemical changes 

 experienced by solid and liquid masses from any enormous 

 compression. It is with the greatest difficulty that our powers 

 of comprehension can conceive the boundaiy line which divides 

 the fluid mass of the interior from the hardened mineral 

 masses of the external surface, or the gradual increase of the 

 solid strata, and the condition of semi-fluidity of the earthy 

 substances these being conditions to which known laws of 

 hydraulics can only apply under considerable modifications. 

 The Sun and Moon, which cause the sea to ebb and flow, most 

 probabfy also affect these subterranean depths. We may sup- 



* Here we must notice the admirable analytical labours of Fourier, 

 Biot, Laplace, Poisson, Duhamel, and Lame". In his Theorie mathema- 

 tique de la, Chaleur, 1835, pp. 3, 428-430, 436, and 521-524 (see also 

 De La Eive's abstract in the Bibliotheque universelle de Geneve), Poisson 

 has developed an hypothesisjtotally different from Fourier's view (Theorie 

 analytique de la Chaleur.} He denies the present fluid state of the 

 Earth's centre ; he believes that " in cooling by radiation to the medium 

 surrounding the Earth, the parts which were first solidified sunk, and 

 that by a double descending and ascending current, the great inequality 

 was lessened which would have taken place in a solid body cooling from 

 the surface." It seems more probable to this great geometer that the 

 solidification began in the parts lying nearest to the centre : " the phe- 

 nomenon of the increase of heat with the depth does not extend to the 

 whole mass of the Earth, and is merely a consequence of the motion of 

 our planetary system in space, of which some parts are of a very dif- 

 ferent temperature from others, in consequence of stellar heat (chaleur 

 stellaire.)" Thus, according to Poisson, the warmth of the water of our 

 .Artesian wells is merely that Avhich has penetrated into the Earth from 

 without ; and the Earth itself " might be regarded as in the same cir- 

 cumstances as a mass of rock conveyed from the equator to the pole in 

 so short a time as not to have entirely cooled. The increase of tem- 

 perature in such a block would not extend to the central strata." The 

 physical doubts which have reasonably been entertained against this 

 extraordinary cosmical view (which attributes to the regions of space 

 that which probably is more dependent on the first transition of matter 

 condensing from the gaseo-fluid into the solid state) will be found col- 

 lected in Poggendorff's^ranoZe/i, bd. xxxix.s. 93-100. 



