58 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 17, 



instruments being proportionate to the difference of their depths. 

 The same law will also hold true for the indications of the ther- 

 mometers placed at depths less than 60 or 70 feet, provided we take 

 the mean annual temperatures indicated by them instead of their 

 temperatures at any particular time. 



Hence, then, omitting always the diurnal and annual variations of 

 temperature, we have the conclusion that the internal temperature 

 of the earth increases as we descend beneath its surface in the same 

 proportion as the depth. This is the exact law as determined by the 

 mathematical analysis of the problem, for all depths which are suffi- 

 ciently small compared with the earth's radius. It might be taken 

 as approximately true for at least 200 or 300 miles, supposing the 

 constitution of the earth to such depths to be similar to that at its 

 surface. Observation has corroborated the truth of the conclusion 

 to all accessible depths, the deviations from the law being considered 

 as due to local causes. 



3. The oscillations of temperature above-mentioned within the 

 range of 60 or 70 feet in this latitude, whether diurnal or annual, are 

 due to the irregular supply of heat received from the sun, especially 

 in the higher latitudes. The continuous supply of heat from the sun 

 during an indefinite period has also prevented the mean internal tem- 

 perature at any point from descending so low as it would have done 

 in the absence of that luminary. Thus the actual temperature of 

 any point within the earth is partly due to the remains of the earth's 

 primitive heat (according to the theory of terrestrial temperature that 

 we are now considering), and partly to the heat received from the sun 

 during the whole term of the earth's refrigeration. Poisson has esti- 

 mated the part of the earth's superficial mean temperature due to the 

 latter cause at about 43° F. in the latitude of Paris, more than 59° F. 

 at the equator, and less than 25° F. at the poles. These results, how- 

 ever, may be hable to considerable uncertainty. No further alteration 

 can take place from this cause. The part of the superficial tempe- 

 rature due to the primitive heat is determined without any similar 

 uncertainty. It is very small, amounting only to about one-twentieth 

 of a degree of Fahrenheit. It must have been constantly diminishing 

 for an immense period of time, and has now approximated so near to 

 its ultimate limit, that if the earth's refrigeration should continue 

 under the same external conditions as at present, it would require, as 

 shown by Poisson, the enormous period of a hundred thousand mil- 

 lions of years to reduce this small fraction to half its actual value. 

 Hence the mean superficial temperature of the earth, and consequently 

 the mean temperature of the atmosphere in contact with the earth's 

 surface, may be considered as sensibly unalterable for all future time, 

 provided always, that the sun, the earth's atmosphere, and the tem- 

 perature of surrounding space shall remain unchanged. 



4. We are here, however, more immediately concerned with past 

 than future changes. We should not be justified in supposing, from 

 what I have stated respecting the slow future variation of the earth's 

 superficial temperature, that it has been equally slow for an equal 

 period of past time ; but still it is highly probable that some millions 



