Dr. Franklin gave a different form to the electric theory of the Aurora, supposing that the electricity which is concerned in the phenomenon passes into the Polar regions from the immense quantities of vapour raised into the atmosphere between the tropics (Exper. and Observ. 1769, p. 43). [Sidenote: Mr. Kirwan’s theory.] Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora Borealis and Australis was occasioned by the combustion of inflammable air kindled by electricity. [Sidenote: Mons. Monge’s.] Mons. Monge proposed the theory that the Auroræ were merely clouds illuminated by the sun’s light falling upon them after numerous reflections from other clouds placed at different distances in the heavens (Leçons de Physique par Prejoulz, 1805, p. 237). [Sidenote: Mons. Libes’.] Mons. Libes propounded a theory that the electric fluid, passing through a mixture of azote and oxygen, produced nitric acid, nitrous acid or nitrous gas, and that these substances, acted upon by the solar rays, would exhibit those red and volatile vapours which form the Aurora Borealis (Traité de Physique, ou Dictionnaire de Physique, par Libes; Rozier’s Journal, June 1790, February 1791, and vol. xxxviii. p. 191). [Sidenote: Mr. Dalton’s.] Mr. Dalton considered the Aurora a magnetic phenomenon whose beams were governed by the magnetism of the earth. He observed that the luminous arches were always perpendicular to the magnetic meridian (Dalton’s Meteorological Observations and Essays, 1793, pp. 54, 153). [Sidenote: Abbé Bertholon’s.] The Abbé Bertholon ascribed the Aurora Borealis to a phosphorico-electric light (Encyc. Méthod. art. Auroræ). [Sidenote: Dr. Thompson concluded the arches to be an optical deception.] Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the observations of Mr. Cavendish and Mr. Dalton, concluded there was no doubt that the arched appearance of the Aurora was merely an optical deception, and that in reality it consisted of a great number of straight cylinders parallel to each other and to the dipping-needle at the place where they were seen. [Sidenote: Artificial Auroræ produced in exhausted tubes.] With many of us (at least it was so in my own case) our first viewed Auroræ have been artificial ones, devised by electricians and having their locus at the Royal Polytechnic in Regent Street or in some scientific lecture-room. The effects in these cases are produced in tubes nearly exhausted by means of an air-pump, and then illuminated by some form of electric or galvanic current. [Sidenote: Tubes described.]