300 Geological Society. 



lapse of time, in the very contemplation of which our minds become 

 bewildered, the law of gravitation underwent no change, and the 

 powers of atomic combination were still performing their office. 



If the phaenomena of geology be coeval with long returning astro- 

 nomical periods (and it is at least impossible to prove the contrary), 

 a question may arise, 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 

 discussed in a recent paper by Mr. Herschel. 



Of all the secular inequalities produced by perturbation, those of 

 the moon alone can produce any visible 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 movements 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 diffusion 

 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 eccen- 

 tricity of the earth's orbit has been for many ages slowly diminish- 

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

 have not yet been calculated. He assumes therefore, hypotheti- 

 cally, 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 phaenomena. 



Several other great modifications in the diffusion of light and 

 heat are involved in this hypothesis, one only of which I will men- 

 tion, as it can be easily explained. It is well known that the place 

 of the apogee and the equinoctial points are both in continual 

 movement ; and after the completion of a long cycle, these points 

 will have travelled through the whole circumference of our orbit ; 

 whence it follows that, during one part of the great astronomical 

 cycle, our summers would coincide with the greatest, and during; 

 another with the least distance from the sun. And these con- 

 ditions, in an orbit of considerable eccentricity, would produce, 

 at one time a climate resembling perpetual spring; at another, 

 the extreme vicissitudes of a burning summer and a rigorous 

 winter. 



Whether influences of this kind ever have caused any con- 

 siderable effects on the climate of different portions of our globe, 

 must, however, still remain in doubt, as the calculations are only 

 founded on analogy. We rejoice, however, to associate our science 



with 



