245 



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 Mecanique Celeste, equation 

 («), 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- 

 tion of the apogee of the earth's orbit, the two hemispheres would 

 alternately be placed in climates of a very opposite nature, the one 

 approaching to a perpetual spring, the other to the extreme vicis- 

 situdes of a burning summer and a rigorous winter ; and that, du- 

 ring periods sufficiently long to impress a corresponding character 

 of the vegetable and perhaps the animal productions of each. 



