300 



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 phenomena. 



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 changes in 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 these lofty speculations, in which man seems to be no longer 

 a worshiper at the portal of Nature's temple, but is allowed to pass 

 within, and to be so far a partaker of her mysteries, as to see with 

 his intellectual eye both the past and the future. 



I believe that the law of gravitation, the laws of atomic affinity, 

 and, in a word, all the primary modes of material action, are as 

 immutable as the attributes of that Being from whose will they 

 derive their only energy. But it is not merely through the simple 

 and unchangeable modes of material action, or through the simple 

 laws by which the parts of material things are bound together, that 

 the works of nature are submitted to our senses. The things 

 we see on the surface of the earth are in a continual state of move- 

 ment and change, of destruction and renovation. They are not 

 merely subject to those fundamental powers, commonly considered 

 as the laws of nature ; but the very powers themselves act under 

 such endless modifications, sometimes combined together, and some- 

 times in conflict, that there follow from them results of indefinite 

 complexity, the very simplest of which are removed far out of the 

 reach of any rigid calculation. 



As the primary laws of matter are immutable, every physical ex- 

 periment tried under the same conditions must end in the same 



