346 Rev. 0. Fisher, On the hypothesis of a [May 30, 



generation of heat (though small) in the water. A like cause 

 must be in operation within the earth, because the forced bodily 

 tide has a different period from the free gravitational oscillation. 

 The distribution of the additional heat from this cause would pro- 

 bably, if calculated, turn out to be different from that arising from 

 the internal friction produced by the tidal couple, which is the 

 source of internal heat contemplated in Prof. Darwin's work. 



It seems then that, unless the ocean tides have been in opera- 

 tion for a length of time exceeding any estimate hitherto suggested, 

 it does not appear probable that any considerable portion of the 

 heat, which according to Darwin's hypothesis of the moon being 

 shed from the earth has been from first to last generated about 

 the earth, can be got rid of by that means. It follows that a 

 largely preponderating amount of it must have accumulated within 

 the earth. This as already remarked must have kept the deeper 

 parts constantly above the temperature of solidification for the 

 pressure, and is an argument in favour of present liquidity. 



But if such is their condition we cannot appeal to the slowness 

 of conduction in a solid earth to account for this great amount of 

 heat not making itself evident at the surface ; which it must have 

 done unless it has been prevented from accumulating faster than 

 it has been generated. There seem to be only three important 

 means of effecting this, viz. (1) conduction through the solidified 

 crust, (2) transference of heat to the surface by volcanic action, 

 (3) the conversion of heat into work against gravity, and against 

 the molecular forces, expended in modifying during geological 

 ages the condition of the crust. 



The first and most obvious mode of escape of heat from the 

 interior, which we now regard as liquid, is by conduction through 

 the solidified crust. I have explained in my Physics of the Earth's 

 Crust*, how it is the latent heat of the layer by which the crust 

 would have been thickened more than, owing to the action of the 

 hot liquid, it is actually thickened, which escapes by conduction 

 through the crust, and that this heat is abstracted from the interior 

 mass and lowers its temperature. I have also shown that the 

 mean fall of temperature of the interior from this cause, consider- 

 ing the store of heat to have been initial f and the time elapsed 



* p. 73. 



t It appears however that this assumption is not necessary, for the whole mass 

 remelted (using the symbols in Physics of the Earth's Crust) is y x Air (r - k) 2 , and 

 it yields up X times that amount of heat. 



This divided by the volume of the interior will give the mean fall of temperature 



!*(?•- 7c) 3 J r-k 



or putting y for -- = 3X7 - nearly, 



as on the hypothesis of the heat being initial. 



