56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 17, 



2. On the Causes which may have produced Changes in the 

 Earth's Superficial Temperature. By W. Hopkins, Esq., 

 M.A., F.R.S., Pres. G.S., and Pres. Cambridge Phil. Soc. 



Part I. 



On the Influence of the Earth's internal Heat, and of the 

 Heat radiating from external Bodies on the Earth's Su- 

 perficial Temperature. 



§ I. Influence of the EartNs Internal Heat. 



1. The problem of the internal temperature of the earth, and, 

 assuming that temperature to have been formerly much greater than 

 at present, the laws according to which its refrigeration would take 

 place, have exercised the powers of the first mathematicians. The 

 problem which has been actually solved, however, is more simple 

 than the one which the earth really presents to us. The simplest 

 conditions which have been assumed, are those of a sphere of very 

 large dimensions, like the earth, consisting of matter of which the 

 power of transmitting heat {the conductive povjer) is supposed to be 

 the same in every part ; also the temperature of the space sur- 

 rounding the sphere {the external temperature) is supposed to remain 

 invariable during the whole term of the sphere's refrigeration. Poisson 

 solved the problem with less restrictive conditions as regards the 

 temperature of the external space, by taking account of the influence 

 of the earth's atmosphere ; but he also assumed, like those who had 

 preceded him, the uniformity of the conductive power throughout 

 the cooling mass. It is not likely that this is really the case with 

 the matter composing our globe, and our ignorance on this point 

 might lead to serious error in the determination of the internal tem- 

 perature at points remote from the earth's surface. But this is 

 more than the mathematician has attempted. He has limited himself 

 to points connected with the temperature at depths beneath the sur- 

 face of his imaginary sphere very small compared with its radius, and 

 after the lapse of an enormous time since the cooling process com- 

 menced. The problem is thus very much simplified, and the results 

 obtained are doubtless approximately the same as those which would 

 result from the supposition of the variability of the conductive power 

 of the mass. It is also important to remark, that the results with 

 which we are here concerned are independent of the primitive tempe- 

 rature of the heated sphere. It may have resulted from causes external 

 or internal, and may have been regular or irregular, provided always 

 the time during which the cooling has been proceeding has been of 

 sufficient duration. 



2. A thermometer placed a few inches below the surface of the 

 ground will indicate a variable temperature, which will usually arrive 

 at its maximum after noon, and its minimum after midnight. These 

 variations are due to the irregular supply of heat from the sun during 

 the twenty-four hours, and are not sensible at a depth greater than 

 about 2 feet. If a second thermometer be placed at this latter 



