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high temperature, but also a gradual refrigeration of the surface of 

 the earth. 



Here however we meet with an unexpected difficulty. If during 

 any period the earth have undergone a sensible refrigeration, it 

 must also have undergone a contraction of its dimensions; and also, 

 as a necessary consequence of a well known mechanical law, an ac- 

 celeration round its axis of rotation. But direct astronomical ob- 

 servations prove that there has been no sensible diurnal acceleration 

 during the last 2000 years ; and therefore, by inverting the steps of 

 the reasoning, we prove — that during that long period there has been 

 no sensible diminution in the mean temperature of the earth. This 

 difficulty does not, however, entirely upset the previous hypothesis: 

 it only proves that the earth had reached an equilibrium of mean 

 temperature before the commencement of good astronomical ob- 

 servations. 



But if, Gentlemen, our speculations are thus limited and guided 

 by the observations of astronomy, we have in part paid back to that 

 exalted science the obligations we owe to it. The great bodies 

 of our system leave behind them no marks to track their pro- 

 gress through the heavens ; and the vast secular periods we can 

 calculate, reaching to ages long anterior to the records of our 

 being, might be mere fictions of the mind which have never had any 

 archetype in nature. But in the phenomena of geology we are 

 carried back, almost at our first step, into times unlimited by any 

 narrow measures of our own; and we exhibit and arrange the monu- 

 ments of former revolutions requiring for their accomplishment per- 

 haps all the secular periods of astronomy. Nor is this all. We show 

 by help of records, not to be misinterpreted, that during this vast 

 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 phenomena 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 : 



