[Sidenote: Dalton’s calculation of 100 miles. Backhouse’s 50 to 100 miles. Prof. Newton, mean 130 miles. Elevation of Auroræ cannot exceed a few miles.] In the article “Aurora Polaris,” Encyc. Brit., edition ix., Dalton is instanced as having calculated the height of an Aurora in the north of England at 100 miles; and Backhouse as having made many calculations, with the result of an average height of 50 to 100 miles. Prof. Newton, too, is quoted for the height of 28 Auroræ (calculated by one observation of altitude and amplitude of an arch) as ranging from 33 to 281 miles, with a mean of 130 miles. It is, however, pointed out that a height of 62 miles above the earth’s surface would imply a vacuum attainable with difficulty, even with the Sprengel pump. This difficulty is then met by a reference to the observed altitude of some meteors, and to a suggestion of Prof. Herschel’s that electric repulsion may carry air or other matter up to a great height. Dr. Lardner (‘Museum of Science and Art,’ vol. x. p. 192) speaks of the height of Auroræ as not certainly ascertained; but considers them atmospheric phenomena scarcely above the region of the clouds, and does not think it probable that their elevation in any case can exceed a few miles. [Sidenote: M’Clintock’s observations. Capt. Ross saw Auroræ on an ice-cliff, which he attributed to electric action.] M’Clintock, after noticing that the beams of the Aurora were most frequently seen in the direction of open water, says that in some cases patches of light could be plainly seen a few feet above a small mass of vapour over an opening in the ice. Captain Ross, in his Antarctic voyage, saw the bright line of the Aurora forming a range of vertical beams along the top of an ice-cliff; and suggested this was produced by electrical action taking place between the vaporous mist thrown upwards by the waves against the berg, and the colder atmosphere with which the latter was surrounded. [Sidenote: Bergman estimates height as 468 miles.] Bergman, from a mean of 30 computations, makes the height of the phenomenon to be 72 Swedish (about 468 English) miles. [Sidenote: Boscovich 825 miles.] Father Boscovich calculated the height of an Aurora Borealis observed on the 16th December, 1727, to have been 825 miles. [Sidenote: Mairan 600 miles. Euler several thousand miles. Dr. Blagden about 100 miles.] Mairan supposed the far greater number of Auroræ to be at least 600 miles above the surface of the earth. Euler assigned them an elevation of several thousands of miles. Dr. Blagden, however, limited their height to about 100 miles, which he supposed to be the region of fireballs—remarking that instances were upon record in which northern lights had been seen to join and form luminous balls, darting about with great velocity, and even leaving a train behind them like common meteors (Phil. Trans. vol. lxxiv. p. 227). [Sidenote: Dalton 150 miles.]