2 Prof. W. Thomson on the Secular Cooling of the Earth. 



article recently published in e Macmillan's Magazine'*, where I 

 have shown that most probably the sun was sensibly hotter a 

 million years ago than he is now. Hence geological specula- 

 tions 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 " Uniformi- 

 tarian " school. A " middle path," not generally safest in scien- 

 tific 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. 



4. Fourier's mathematical theory of the conduction of heat is 

 a beautiful working out of a particular case belonging to the 

 general doctrine of the "Dissipation of Energy" j\ A charac- 

 teristic of the practical solutions it presents is, that in each case 

 a distribution of temperature, becoming gradually equalized 

 through an unlimited future, is expressed as a function of the 

 time, which is infinitely divergent for all times longer past than 

 a definite determinable epoch. The distribution of heat at such 

 an epoch is essentially initial, that is to say, it cannot result from 

 any previous condition of matter by natural processes. It is, 

 then, well called an "arbitrary initial distribution of heat," in 

 Fourier's great mathematical poem, because it could only be 

 realized by the action of a power able to modify the laws of dead 

 matter. In an article published about nineteen years ago in the 

 f Cambridge Mathematical Journal 'J, I gave the mathematical 

 criterion for an essentially initial distribution ; and in an inau- 

 gural essay, " De Motu Caloris per Terrse Corpus," read before 

 the Faculty of the University of Glasgow in 1846, I suggested, 

 as an application of these principles, that a perfectly complete 

 geothermic survey would give us data for determining an initial 

 epoch in the problem of terrestrial conduction. At the meeting 

 of the British Association in Glasgow in 1855, I urged that 

 special geothermic surveys should be made for the purpose of 

 estimating absolute dates in geology; and I pointed out some 

 cases, especially that of the salt-spring borings at Creuznach in 

 Rhenish Prussia, in which eruptions of basaltic rock seem to 

 leave traces of their igneous origin in residual heat§. I hope 



* March 1862. 



t Proceedings of the Royal Society of Edinburgh, February 1852, " On 

 a Universal Tendency in Nature to the Dissipation of Mechanical Energy." 

 Also, " On the Restoration of Energy in an Unequally Heated Space," Phil. 

 Mag. S. 4.vol.v. p. 102. 



% February 1844, "Note on Certain Points in the Theory of Heat." 



§ See British Association Report of 1855, Glasgow Meeting. 



