As the general result of spectrum work on the Aurora up to the present time, we seem to have quite failed in finding any spectrum which, as to position, intensity, and general character of lines, well coincides with that of the Aurora. Indeed, we may say we do not find any spectrum so nearly allied to portions even of the Aurora-spectrum, as to lead us to conclude that we have discovered the true nature of one spectrum of the Aurora (supposing it to comprise, as some consider, two or more). The whole subject may be characterized as still a scientific mystery—which, however, we may hope some future observers, armed with spectroscopes of large aperture and low dispersion, but with sufficient means of measurement of line positions, and possibly aided by photography, may help to solve. The singular absence of Auroræ has, for some time past, given no opportunity in that direction. May some of my readers be more fortunate in obtaining opportunities of viewing the glorious sky-fires, and assist to unravel so interesting a paradox! APPENDICES. APPENDIX A. REFERENCES TO SOME WORKS AND ESSAYS ON THE AURORA. (Most of these are cited in the ‘Edinburgh Encyclopædia’ and the ‘Encyclopædia Britannica.’) Musschenbroek, Instit. Phys. c. 41. ‘Trai. Phys. et Hist. de l’Aurore Boréale,’ par M. de Mairan. Paris, 1754. Beccaria, ‘Dell’Elettricismo Artif. e Nat.’ p. 221. Smith’s ‘Optics,’ p. 69. D’Alembert’s ‘Opuscules Mathématiques,’ vol. vi. p. 334. ‘Philosophical Transactions’ as under:— Vol. Pages 1716 406 1717 584, 586 1719 1099, 1101, 1104, 1107 1720 21 1721 180, 186 1723 300 1724 175 1726 128, 132, 150 1727 245, 301 1728 453 1729 137 1730 279 1731 53-55 1734 243, 291 1736 241 1740 368 1741 744, 839, 840, 843 1750 319, 345, 346, 499 1751 39, 126 1752 169 1753 85 1762 474, 479 1764 326, 332 1767 108 1769 86, 307 1770 532 1774 128 1781 228 1790 32, 47, 101 ‘Miscell. Berolinens.’ 1710, vol. i. p. 131. ‘Comment. Petrop.’ tom. i. p. 351, tom. iv. p. 121. ‘Acta Petrop.’ 1780, vol. iv. p. 1. ‘Mem. Acad. Paris,’ 1747, pp. 363, 423; 1731; 1751. ‘Mem. Acad. Berl.’ 1710, vol. i. p. 131; 1747, p. 117. Schwed. ‘Abhandlungen,’ 1752, p. 169; 1753, p. 85; 1764, pp. 200, 251. Bergman, ‘Opusc.’ vol. v. p. 272. ‘Americ. Trans.’ vol. i. p. 404. ‘Mém. de Mathémat. et Phys.’ tom. viii. p. 180. Rozier, vol. xiii. p. 409; vol. xv. p. 128; vol. xxxiii. p. 153. Franklin’s Works, vol. ii. Weidler, ‘De Aurora Boreale.’ 4to. Nocetus, ‘De Iride et Aurora Boreale, cum Notis Boscovisch.’ Rome, 1747. Chiminello, ‘Mem. Soc. Ital.’ vol. vii. p. 153.