ology_, 1885.]

[Footnote 901: See for a popular account of the theory, Sir R. Ball's
_The Cause of an Ice Age_, 1892.]

[Footnote 902: See A. Woeikof, _Phil. Mag._, vol. xxi., p. 223.]

[Footnote 903: _The Ice Age in North America_, London, 1890.]

[Footnote 904: _Phil. Trans._, vol. lxviii., p. 783.]

[Footnote 905: _Comptes Rendus_, t. lxxvi., p. 954.]

[Footnote 906: _Potsdam Publ._, Nos. 22, 23.]

[Footnote 907: _Phil. Trans._, vol. clxxxii., p. 565; _Adams Prize Essay
for 1893._]

[Footnote 908: _Denkschriften Akad. der Wiss. Wien_, Bd. lxiv.; quoted
by Poynting. _Nature_, vol. lxii., p. 404.]

[Footnote 909: _Report on the Geodetic Survey of S. Africa_, 1894.]

[Footnote 910: _Nature_, vol. lxii., p. 622; Hollis, _Observatory_, vol.
xxiii., p. 337; Poincare, _Comptes Rendus_, July 23, 1900.]

[Footnote 911: _Astr. Nach._, No. 2,228.]

[Footnote 912: Young's _Gen. Astr._, p. 601.]

[Footnote 913: _Astr. Constants_, p. 195.]

[Footnote 914: The second volume was published at Gottingen in 1802.]

[Footnote 915: _Ueber Rillen auf dem Monde_, p. 13. _Cf. The Moon_, by
T. Gwyn Elger, p. 20. W. H. Pickering, _Harvard Annals_, vol. xxxii., p.
249.]

[Footnote 916: _The Moon_, p. 73.]

[Footnote 917: _Selen. Fragm._, Th. ii., p. 399.]

[Footnote 918: _Astr. Nach._, No. 263 (1834); _Pop. Vorl._, pp. 615-620
(1838).]

[Footnote 919: _Outlines of Astr._, par. 431.]

[Footnote 920: _Month. Not._, vol. xxv., p. 61.]

[Footnote 921: _Month. Not._, vol. xxv., p. 264.]

[Footnote 922: _Astroph. Jour._, vol. vi., p. 422.]

[Footnote 923: _Harvard Annals_, vol. xxxii., p. 81.]

[Footnote 924: _Astr. and Astrophysics_, vol. xi., p. 778.]

[Footnote 925: Neison, _The Moon_, p. 25.]

[Footnote 926: _Knowledge_, vol. xvii., p. 85.]

[Footnote 927: Neison, _The Moon_, p. 104.]

[Footnote 928: The combination of a uniform rotational with an unequal
orbital movement causes a slight swaying of the moon's globe, now east,
now west, by which we are able to see round the edges of the averted
hemisphere. There is also a "parallactic" libration, depending on the
earth's rotation; and a species of nodding movement--the "libration in
latitude"--is produced by the inclination of the moon's axis to her
orbit, and by her changes of position with regard to the terrestrial
equator. Altogether, about 2/11 of the _invisible_ side come into view.]

[Footnote 929: _Cel. Objects_, p. 58 (4th ed.).]

[Footnote 930: _Astr