| | | | | From 469·4 } | | to 462·9 } 3 stronger and 4 very faint iron-lines. | +----------------------------------------------------------+ Yet this agreement, though remarkable, can only be considered as complete proof of the presence of iron-vapour in the atmosphere when we shall have succeeded in showing by observation analogous modifications of the relative conditions of brilliancy in the iron-spectrum by alterations of temperature and density; and in this way explain the appearance of relatively very faint iron-lines in the Aurora-spectrum, or, on the other hand, the absence of the most intense lines. It will meanwhile remain far more in accordance with probability to regard the _Aurora-spectrum as a modification of the air-spectrum_; since we are already aware, in the case of gases, of the alteration of the spectra by conditions of temperature and pressure; and an agreement, at any rate, quite as certain between the spectrum in question and the spectra of atmospheric gases has been proved above. [I am indebted to Miss Annie Ludlam for a translation from the German of the above Memoir.—J. R. C.] FOOTNOTES [1] βόθυνος, a hollow. [2] πίθος, a cask. [3] χάσμς, a chasm. [4] M. Lemström (Swedish Expedition, 1868) concludes that the corona of the Aurora Borealis is not entirely a phenomenon of perspective, but that the rays have a true curvature, that they are currents flowing in the same direction and attract each other. There is also an account [_antè_, p. 16] of an Aurora at Melville Island (Parry’s first voyage), in which two arches were seen curving towards each other. [5] A brilliant display in December 1870, on the east coast of Sicily, was followed by very violent storms, with the overflow of the Tiber and the flooding of Rome. [6] Some curious instances have been recently (January 1879) given in the ‘Times’ of such electric phenomena, comprising, amongst others, gas lighted by the finger in Canada, points of flame seen on the ironwork of Teignmouth Bridge, and similar points seen on the alpenstocks and axes of a party making a mountain ascent in Switzerland. [7] On the occasion of the Aurora of September 24, 1870, Dr. Allnatt says, “the air seemed literally alive with the unwonted phosphorescence.” [8] See, however, Dr. Schuster’s article “On the Spectra of Lightning,” Phil. Mag. May 1879, p. 316. [9] The proof from occulted stars merely goes to the fact that the moon possesses no atmosphere _appreciable in that way_. It may still be a question whether there does not exist something of the kind, lying low and close to the surface, and possibly of a rarefied character, which would scarcely make itself visible by its effects in occultations. Cloud-vapour might form in an atmosphere of inconsiderable density.