[217] Reader, February 10, 1866.
[218] In a former paper I considered the effects of another cause, viz., the melting of polar ice resulting from an increase of the Obliquity of the Earth’s Orbit.—Trans. Glasgow Geol. Soc., vol. ii., p. 177. Phil. Mag., June, 1867. See also [Chapter XXV.]
[219] Phil. Mag. for November, 1868, p. 376.
[220] Phil. Mag., November, 1868.
[221] “Origin of Species,” chap. xi. Fifth Edition.
[222] Lieutenant-Colonel Drayson (“Last Glacial Epoch of Geology”) and also Mr. Belt (Quart. Journ. of Science, October, 1874) state that Leverrier has lately investigated the question as to the extent of the variation of the plane of the ecliptic, and has arrived at results differing considerably from those of Laplace; viz., that the variation may amount to 4° 52′, whereas, according to Laplace, it amounts to only 1° 21′. I fear they are comparing things that are totally different; viz., the variation of the plane of the ecliptic in relation to its mean position with its variation in relation to the equator. Laplace estimated that the plane of the ecliptic would oscillate to the extent of 4° 53′ 33″ on each side of its mean position, a result almost identical with that of Leverrier, who makes it 4° 51′ 42″. But neither of these geometricians ever imagined that the ecliptic could change in relation to the equator to even one-third of that amount.
Laplace demonstrated that the change in the plane of the ecliptic affected the position of the equator, causing it to vary along with it, so that the equator could never possibly recede further than 1° 22′ 34″ from its mean position in relation to the ecliptic (“Mécanique Céleste,” vol. ii., p. 856, Bowditch’s Translation; see also Laplace’s memoir, “Sur les Variations de l’Obliquité de l’Écliptique,” Connaissance des Temps for 1827, p. 234), and I am not aware that Leverrier has arrived at a different conclusion.
[223] Memoir on the Secular Variations of the Elements of the Orbits of the Planets, “Smithsonian Contributions to Knowledge,” vol. xvii.
[224] “Smithsonian Contributions to Knowledge,” vol. ix.
[225] “Distribution of Heat on the Surface of the Globe,” p. 14.