[13] The Tides, by G.H. Darwin, p. 333.
[14] Ikaromenippus; or, Above the Clouds. Prof. D.C. Brown's translation.
[15] The Moon, a Full Description and Map of its Principal Features, by Thomas Gwyn Elger, 1895.
Those who desire to read detailed descriptions of lunar scenery may consult, in addition to Mr. Elger's book, the following: The Moon, considered as a Planet, a World, and a Satellite, by James Nasmyth and James Carpenter, 1874; The Moon, and the Condition and Configurations of its Surface, by Edmund Neison, 1876. See also Annals of Harvard College Observatory, vol. xxxii, part ii, 1900, for observations made by Prof. William H. Pickering at the Arequipa Observatory.
[16] The discovery of free hydrogen in the earth's atmosphere, by Professor Dewar, 1901, bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. Since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. In a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.
[17] Comptes Rendus, June 26, July 3, 1899.
[18] The Tides, by G.H. Darwin, chapter xvi.
[19] Annals of Harvard College Observatory, vol. xxxii, part ii, 1900.
[20] Comptes Rendus, June 23, July 3, 1899.
[21] In our latitudes, planets are never seen in the northern quarter of the sky. When on the meridian, they are always somewhere between the zenith and the southern horizon.