1832.] Progress of Geological Science. 517 



whether some of the first difficulties we meet with (such as those connected with 

 the transport of diluvial gravel, and the gradual diminution of temperature) may 

 not be attributed rather to effects of planetary perturbation than to any change 

 in the internal condition of the earth. This question has been admirably discuss- 

 ed in a recent paper by Mr. Herschel. 



M Of all the secular inequalities produced by perturbation, those of the moon 

 alone can produce any \isible effects upon the tidal level. The lunar inequalities 

 considered are of two kinds — change of mean distance, and change of eccentricity. 

 Both are confined within narrow determined limits ; and Mr. Herschel shows, 

 by actual calculation, that they could not have produced any of the great move- 

 ments contemplated in geology. 



" The planetary perturbations of the orbit of the earth are next considered, and 

 the influences they may have produced on the diffusion of light and heat. The 

 secular variation of obliquity is too small to have ever caused any sensible effect 

 on our climates : but he proves, by direct calculation, that the mean annual diffu- 

 sion of solar light and heat varies inversely as the minor axis of the orbit ; or, in 

 other words, increases or diminishes with the increase or diminution of eccentricity. 

 Now, as a matter of fact, the eccentricity of the earth's orbit has been for many ages 

 slowly diminishing, and is now very small ; but the limits of its secular variation 

 have not yet been calculated. He assumes, therefore, hypothetically, that the 

 eccentricity of our orbit may once have been as great as that of some of the inferior 

 and superior planets; and on that supposition, he proves, that the slow diminution 

 of eccentricity may have produced a gradual change of climate, of the very 

 kind indicated by geological pliaenomena. 



"Into the solution of the great problem of the heavenly bodies, there enter only 

 a few simple and unchangeable mechanical elements, and the conclusions are of a 

 simplicity corresponding to the simplicity of the premises. All the celestial move- 

 ments return into themselves ; and even the most complex of the deviations pro- 

 duced by mutual perturbation, are confined within narrow limits, and are com- 

 pleted in secular periods. The solution of this problem is incontestably the great- 

 est triumph of exact science. But with what semblance of physical truth can we 

 apply such mathematical results as these to the great pliaenomena of geology — 

 where the combinations are mutable and indefinite — where we have no vestige of 

 retiunin"- periods— and where the fixed elements of force are either unknown or 

 imperfectly comprehended ? 



If all the complex groups of crystalline and stratified rocks ; if, in a word, all 

 the material things existing on the surface of the globe, be bound to each other by 

 laws like those which govern the movements of the heavenly bodies — then every 

 material combination we now see must re-appear with all its complicated relations 

 after the lapse of some long period of time. But would not such a supposition be 

 now regarded as the mere wnntonness of hypothetical extravagance ? And let it 

 not be said, that it is only in the greater combinations on the surface of the earth 

 that we are to look for returning cycles. Great and small have no meaning, except 

 in reference to us and our conceptions. The earth is an atom in comparison with 

 the visible creation ; and all we now behold may be but as an atom in comparison 

 of that which is unseen ; and the meanest combinations of material things submitted 

 to our senses propagate their influence through all space co-extensive with gravita- 

 tion, and play their part in keeping up the stability of the universe. 



To the supreme Intelligence, indeed, all the complex and mutable combinations we 

 heboid, may be but the necessary results of some simple law, regulating every ma- 



