212 SEE— TEMPERATURE, SECULAR COOLING [April 20. 



basins, and continental platforms ; and we probably are not justified 

 in ascribing any sensible part of these inequalities of level to deep 

 seated shrinkage of particular segments. For we have seen that the 

 general shrinkage is quite incapable of producing such large effects, 

 and there is still less reason why certain segments should shrink so 

 much more than others. The average depth of 2.5 miles for the 

 Pacific Ocean can not well be explained by shrinkage. It appears 

 therefore that the great inequalities of the earth's surface resulted 

 from the formative processes involved in the detachment of the moon, 

 while the smaller inequalities such as mountains, high plateaus, and 

 deep ocean troughs and abysses have been produced mainly by earth- 

 quakes acting in the layer just beneath the crust. In this way we may 

 legitimately explain all the leading features of the earth's surface; 

 but it is obvious that we cannot give the details involved in the for- 

 mation of each continent and ocean, because there is no way of re- 

 tracing this early history of our planet. 



II. On the Secular Cooling of the Earth. 

 § 8. Analytical Theory of the Propagation of Heat in Solid Bodies 



Applied to the Secular Cooling of th€ Terrestrial Globe. 



This subject was first treated with characteristic penetration by 

 Fourier, in a memoir entitled " Le Refroidissement Seculaire du 

 Globe Terrestre," communicated to the Philomathique Society of 

 Paris in 1820 (cf. " Oeuvres de Fourier," Tome II, pp. 271-288). It 

 had, however, already been touched upon in the " Theorie Analytique 

 de la Chaleur " (chap. Ix., § II., p. 427 et seq.), which, after a delay 

 of some 15 years, was finally published in 1822. 



Lord Kelvin's famous discussion of the "Secular Cooling of the 

 Earth " (Appendix D, Thomson and Tait's " Nat. Phil.," Vol. I., 

 Part II.) is based on Fourier's methods ; but the constants were care- 

 fully determined from new experiments on underground temperature, 

 which gave the conductivity of average rock, and thus made possible 

 important estimates of the age of the earth. Rev. O. Fisher has also 

 treated this and many other questions in his valuable and suggestive 

 work on the "Physics of the Earth's Crust" (second edition). 



The mathematical treatment consists in determining by Fourier's 

 fundamental equations the actual temperature at any point in a solid 



