1851.] HOPKINS ON CHANGES OF CLIMATE. 59 



of centuries must have elapsed since the mean superficial temperature 

 could have been greater by a single degree than at present, from the 

 operation of the causes we are now discussing, assuming the per- 

 manence of the conditions above-mentioned. However imperfect our 

 geological chronology may be, these enormous periods of time seem 

 almost to preclude the possibility of applying this theory to account 

 for the changes of temperature of which we have evidence in the 

 more recent geological periods. 



5. There is also another result for which we are indebted to the ma- 

 thematical solution of the problem of terrestrial temperature. The 

 effect of internal heat on the earth's superficial temperature must 

 have been, during a long period, constantly decreasing, as already 

 stated. Also the rate at which the internal heat decreases as the depth 

 beneath the earth's surface increases, must, for a like period of time, 

 have been constantly decreasing. The present effect of the internal 

 heat, as above-stated, is about ^th of a degreee ; and the rate of 

 increase of temperature below the earth's surface is about 1° F. 

 for every 60 feet. Now we learn from mathematical investigation 

 that this effect of the internal heat on the superficial temperature 

 bears a constant ratio to the rate of increase just mentioned. Con- 

 sequently, knowing this ratio at the present time, we can ascertain 

 the rate at which the temperature must have increased in descending, 

 at any past geological epoch at which the effect of the earth's internal 

 heat on the superficial temperature was of any assigned amount. 

 Thus, when the superficial temperature was raised 1° F., twenty times 

 the present amount, by this cause, the descending rate of increase 

 must have been twenty times as great as at present, about 20° F. for 

 every 60 feet ; and, if the superficial temperature were thus raised 

 about 10° F., the temperature at the depth of 60 feet would, according 

 to the same law, exceed 200° F., and all but surface-springs would be 

 springs of boiling water. This physical state of our planet would 

 scarcely perhaps be deemed consistent with the conditions of animal 

 life at the more recent geological epochs. 



6. Formerly it was generally supposed that the only changes of 

 climatal conditions which could be recognized by the geologist, were 

 changes from a higher to a lower general temperature on the earth's 

 surface. More accurate geological research, however, has now shown 

 that there has been no such regular descending progression, but that 

 these changes have been to a considerable extent of an oscillatory 

 character, and so far as they may be thus characterized, they cannot 

 of course be accounted for by the earth's internal heat. So far as 

 the effectiveness of this cause on climatal conditions can be recognized, 

 it can only be with reference to periods which, according to the or- 

 dinary modes of estimating time, must be considered as of enormous 

 antiquity, whatever portion of the great series of geological events 

 might belong to them. We must manifestly seek for other causes to 

 account for the changes of temperature which mark the more recent 

 geological periods. 



