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XV.— Ow the Secular Cooling of the Earth. By Professor William Thomson, 



LL.D., F.R.S., F.R.S.E. (Plate VIII.) 



(Read 2Sth April 1862.) 



1. For eighteen years it has pressed on my mind, that essential 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 annihilation) 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 commence- 

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

 ing from age to age. 



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

 published in " Macmillan's Magazine," * where I have shown that most pro- 

 bably the sun was sensibly hotter a million years ago than he is now. Hence, 

 geological speculations assuming somewhat greater extremes of heat, more violent 

 storms and floods, more luxuriant vegetation, and hardier and coarser grained 

 plants and animals, in remote antiquity, are more probable than those of the 

 extreme quietist, or " uniformitarian" school. A " middle path," not generally 

 safest in scientific speculation, seems to be so in this case. It is probable that 

 hypotheses of grand catastrophes destroying all life from the earth, and ruining 

 its whole surface at once, are greatly in error ; it is impossible that hypotheses 

 assuming an equability of sun and storms for 1,000,000 years, can be wholly 

 true. 



* March 1862. 

 VOL. XXIII. PART I. 2 X 



