382 Mr. 0. Fisher on the Thermal Conditions and 



Sir Wyville Thomson remarks that " it is not easy to see 

 why the temperature of the earth's crust, under a widely ex- 

 tended and practically permanent ice-sheet of great thickness, 

 should ever fall below the freezing-point," Our first inquiry 

 therefore shall be : — 



(1) If a level sheet of ice rest upon the earth, its upper sur- 

 face being maintained at a uniform mean temperature lower 

 than the freezing-point , to find the thickness beyond which melt- 

 ing at the lower surface must take place. 



It is assumed that the form of the ice- cap is permanent ; so 

 that we may consider that the masses, whether of rock or ice, 

 have arrived at that state in which the temperature is also 

 permanent. Probably any horizontal movement in the ice, 

 so long as the thickness at any place remains unaltered, will 

 only very slightly affect this supposition — because, if there be 

 sliding of the parts one over another, ice abstracted by hori- 

 zontal movement will be replaced by ice at the same tempe- 

 rature. 



We know that there is a continual flow of heat out of the 

 earth. This flow is such that it is found to raise the tempe- 

 rature of the strata of the earth by about -^ of a degree Fahr. 

 per foot on descending. This average increase prevails equally 

 wherever observations have been taken. Even at Yakoutzk, 

 where the ground is perpetually frozen beyond the depth 

 pierced, the same rate obtains. This may therefore be ad- 

 mitted as an empirical fact. But the following reasoning may 

 perhaps be accepted as explanatory of this uniformity of rate. 



If the temperature of the surface of the earth were suddenly 

 lowered at any place, the flow of heat to the surface would by 

 that means be temporarily increased, and therefore also the 

 rate of increase near the surface. But since the general in- 

 ternal temperature is no greater below that locality than else- 

 where, this increased flow near the surface must take place at 

 the expense of the superficial strata. Hence the isogeotherms 

 near the surface, though temporarily brought closer together, 

 will shortly begin to separate again, until the rate of increase 

 falls to its normal value. At this juncture the flow of heat at 

 the surface at the place in question will become equalized to 

 that which obtains elsewhere. And since the supply of heat 

 from the interior is the same hero as elsewhere, the rate can 

 fall no lower, and will afterwards continue equal to that which 

 obtains in other regions. It follows that the permanent rate 

 of increase of temperature near the surface of the earth will 

 be everywhere the same, and the flow of heat also, whatever 

 the temperature of the surface may be. This is what obser- 

 vation shows to be the case. 



