THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



JANUARY 1863. 



I. On the Secular Cooling of the Earth. 

 By Professor William Thomson, LL.D., F.R.S., F.R.S.E.* 



[With a Plate.] 

 1. lilOR eighteen years it has pressed on my mind, that essen- 

 -L tial principles of thermo-dynamics have been overlooked 

 by those geologists who uncompromisingly oppose all paroxysmal 

 hypotheses, and maintain not only that we have examples now 

 before us, on the earth, of all the different actions by which its 

 crust has been modified in geological history, but that these 

 actions have never, or have not on the whole, been more violent 

 in past time than they are at present. 



2. It is quite certain the solar system cannot have gone on, 

 even as at present, for a few hundred thousand or a few million 

 years without the irrevocable loss (by dissipation, not by annihi- 

 lation) of a very considerable proportion of the entire energy 

 initially in store for sun heat, and for plutonic action. It is 

 quite certain that the whole store of energy in the solar system 

 has been greater in all past time than at present; but it is 

 conceivable that the rate at which it has been drawn upon and 

 dissipated, whether by solar radiation, or by volcanic action in 

 the earth or other dark bodies of the system, may have been 

 nearly equable, or may even have been less rapid, in certain 

 periods of the past. But it is far more probable that the secular 

 rate of dissipation has been in some direct proportion to the total 

 amount of energy in store, at any time after the commencement 

 of the present order of things, and has been therefore very slowly 

 diminishing from age to age. 



3. I have endeavoured to prove this for the sun's heat, in an 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiii. 

 part 1. Communicated by the Author. 



Phil. Mag. S. 4. Vol. 25. No. 165. Jan. 1863. B 



