Geological Society. 137 



rage but the extreme rise and fall of the tides. Such an increase, 

 however, he regards as necessarily limited, so as to be incapable of 

 producing such an enormous increase of tides as would account for 

 any of the greater diluvial phenomena, though possibly cases of 

 great local devastation in estuaries and confined channels would 

 arise, and the outlines of the continents, in particular parts of their 

 coasts, might be materially modified by such increased occasional 

 action. No change in the earth's orbit within the limits of possibility 

 would produce any material change in the solar tides. 



He next considers the effect of planetary perturbation on the 

 earth's orbit, and, dismissing the variation of the obliquity of 

 the ecliptic, which is known to be confined within very narrow- 

 limits, he regards the excentricity as the only element whose 

 variation can possibly have any effect of the kind in view ; 

 and that by affecting, first, the mean, and secondly, the extreme 

 quantities of solar heat received by the earth in its annual revolu- 

 tion, and at the different seasons of the year. First, with respect 

 to the mean quantity, he announces as a consequence of geome- 

 trical reasoning, the following theorem : That the mean annual 

 amount of heat and light received from the sun by the earth, is inversely 

 proportional to the minor axis of the ellipse it describes at different 

 epochs. And since the orbit of the earth is actually, and has 

 been for ages, beyond the records of history, becoming less ellip- 

 tic, and the minor axis consequently increasing, it follows that 

 the mean temperature of its surface is on the decrease. The 

 orbit being now very nearly a circle, this decrease cannot go much 

 further; but should it ever have been very elliptic, the mean tem- 

 perature must have been sensibly greater than at present. The au- 

 thor regards the limits within which the earth's excentricity is con. 

 fined, as (although calculable; not actually known ; and he denies 

 in particular that the theorem demonstrated by Laplace, in the 

 57th article of the Second Book of the Mdcam'que Celeste, equation 

 (M), which is usually cited as proving the narrowness of such limits, 

 affords any ground for that conclusion in the case of the earth's 

 orbit, however it may do so for those of the great preponderant 

 planets. 



Under this uncertainty he considers himself authorized to assume, 

 that excentricities actually existing in the orbits, both of superior 

 and inferior planets, may not be impossible in that of the earth ; and 

 admitting this, he calculates the mean and extreme amounts of solar 

 radiation in an orbit so circumstanced. The mean amount he finds 

 to exceed the present by about three per cent, a quantity apparently 

 small ; but he adduces considerations tending to show, that on cer- 

 tain suppositions not impossible or improbable in themselves, this 

 per-centage on the whole quantity of solar heat may have influ- 

 enced our climates to as great an extent as geological indications 

 appear to require. 



Considering next the extreme effects of such a state of things, and 

 adopting a view taken by Mr. Lyell in his Geology, he shows that 

 by reason of the precession of the equinoxes combined with the mo- 



N. S. Vol. 9. No. 50. Feb. 1831. T tion 



