INTERNAL HEAT OF THE EARTH. 173 



regai ding the motion of heat in homogeneous metallic sphe 

 roids, must be applied with much caution to the actual chai> 

 acter of our planet, considering our present imperfect knowl- 

 edge 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 pow- 

 ers of comprehension can conceive the boundary line which di- 

 vides 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 probably also affect these subterranean depths. We 

 may suppose that the periodic elevations and depressions of 

 the molten mass under the already solidified strata must have 

 caused inequalities in the vaulted surface from the force of 

 pressure. The amount and action of such oscillations must, 

 however, be small ; and if the relative position of the attract- 

 ing cosmical bodies may here also excite " spring tides," it is 

 certainly not to these, but to more powerful internal forces, 

 that we must ascribe the movements that shake the Earth's 

 surface. There are groups of phenomena to whose existence 

 it is necessary to draw attention, in order to indicate the 

 universality of the influence of the attraction of the Sun and 

 Moon on the external and internal conditions of the Earth, 

 however little we may be able to determine the quantity of 

 this influence. 



According to tolerably accordant experiments in Artesian 

 wells, it has been shown that the heat increases on an average 

 about 1 foi every 54*5 feet. If this increase can be reduced 



are of a very different temperature from others, in consequence of stel- 

 lar heat (chaleur stellaire)." Thus, according to Poisson, the warmth 

 oi' the water of our Artesian wells is merely that which has penetrated 

 iuto the Earth from without; and the Earth itself '* might be regarded 

 as in the same circumstances 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 temperature 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 attribute! 

 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 

 etate) will be found collected ill Poggendorf 's Annalen, bd. xxxix., 

 93-100. 



