AUSCULTATION

Aurora Borealis having been observed in England from the year 1621 to 1707. Celsius says expre ly that the oldest inhabitants of Upsala considered the phenomenon a great rarity before 1716. In the month of March in that year a very splendid one appeared in England, and by reason of its brilliancy attracted universal attention. It has been described by Dr. Hally in the Phil. Trans., No. 347. Since then, the meteor has been much more common. A complete account of all the appearances of Auroras recorded previous to 1754 may be found in the work of Mairan, " Praite de 1' Aurore Boreale." The Aurora is not confined to the Northern hemisphere, similar appearances being observed in high southern latitudes. An Aurora was witne ed by Don Antonio d'Ulloa at Cape Horn in 1745; one appeared at Cuzco in 1744; and another is described by Mr. Forster( who accompanied Captain Cook in his last voyage round the world), which was seen by him in 1773, in latitude 58° south, and resembled, entirely, those of the northern hemisphere, excepting that the light exhibited no tints, but was of a clear white. Similar testimony is given by subsequent navigators. There is great difficulty in determining the exact height of the Aurora Borealis above the earth, and accordingly the opinions given on this subject by different observers are widely discordant. Mairan supposed the mean height to be 175 French leagues. Bergman says 460 miles, and Euler several you sand miles. From the comparison of a number of observations of an Aurora that appeared in March 1826, made at different places in the north of England and south of Scotland, Dr. Dalton, in a paper preBf;nted to the Roj-al Society, computed its height to be about 100 miles. But a calculation of this sort, in which it is of nece ity supposed that the meteor is seen in exactly the same place by the different observers, is subject to very great uncertainty. The observations of Dr. Richardson, Franklin, Hood, Parry, and others seem to prove that the place of the Aurora is far within the limits of the atmosphere, and scarcely above the region of the clouds; in fact, as the diurnal i:otation of the earth produces no change in its apparent position, it must nece arily partake of that motion, and consequently be regarded as an atmospherical phenomenon. No satisfactory theory has yet been given of the cause of the polar lights. Mairan ascribed the phenomenon to the sun's atmosphere; Euler, to particles of the earth's atmosphere driven beyond its limits by the impulse of solar light. Beccaria, Canton, Franklin, and others refer it to electricity, an agent to whose mysterious influence all the inexplicable phenomena of meteorology are conveniently ascribed. An absurd theory proposed by M. Libes (Dictionnaire de Physique) formerly met with considerable favour. He had observed that on pa ing an electric spark through a compound of oxygen and nitrogen, nitrous vapours of a reddish colour are produced. He therefore supposed that the higher regions of the atmosphere near the pole contain little or no hydrogen; and that consequently the discharges of electricity, which, by producing a combination of oxygen and hydrogen, form water in the lower strata, in the more elevated strata produce nitrous vapours, which constitute the polar lights. That some connexion subsists between the Aurora and magnetism, or rather electricity, which is now regarded as the primary cause of magnetism, is made certain by the fact that during the continuance of brilliant Auroras the magnetic needle is generally disturbed, sometime! violently agitated. The air at the same time is often observed to be highly charged with electric matter. An experiment contrived by M. Canton also seems to indicate an electric origin. If a gla tube be partially exhausted of air, hermetically sealed, and applied to the conductor of an electric machine, the whole tube is illuminated from end to end, and continues luminous for a considerable time after it has been removed from the conductor. If, after this, the tube be drawn through the hand, the light will be remarkably intense through its whole length; and if it is grasped in both hands, near the extremities, str or.g flashes of light will dart from one end to the other, and continue many hours without fresh excitation. The only conclusion which, iathe present state of our knowledge, we are warranted in deducing, is, that the Aurora Borealis must be ascribed to the agency of electricity in the upper regions of the atmosphere: in what way the excitement is produced, it remains for future discoveries to make known. The most systematic series of observations which we po e on the subject, was made by the Rev. James Farquharson, of Aberdeen shire, in 1829, with an apparatus furnished by the Royal Society, and of which an account is published in the Transactions for 1830. [s. 122]