JOURNAL OF THE FRANKLIN INSTITUTE OF THE State of Pennsylvania, DEVOTED TO THE MECHANIC ARTS, MANUFACTURES, GENERAL SCIENCE, AND THE RECORDING OF AMERICAN AND OTHER PATENTED INVENTIONS. JANUARY, 1834. TO THE COMMITTEE ON PUBLICATIONS. Observations on the Disturbance in the Direction of the Horizontal Needle, during the occurrence of the Aurora of July 10th, 1833. By A. D. BACHE, Prof. of Nat. Philos. and Chem. in the University of Pennsylvania. (WITH A COPPERPLATE.) GENTLEMEN,-In the number of this Journal for July last, I gave an account of a disturbance in the direction of the horizontal needle, during the occurrence of the Aurora Borealis, on the 17th of May. On the occasion of the brilliant aurora of the 10th of July, I had a remarkably favourable opportunity of observing a similar disturbance, and as the subject is one which is considered as requiring further observations for its elucidation, I send you an account of those made on the evening referred to. The needles with which the observations were made, are the same which were referred to in my note of observations made in May. Their places had remained unchanged from that time; the horizontal needle out of doors, under a small wooden enclosure; the horizontal needle within doors on a table placed against a partition wall in my study; the dipping needle out of doors in a small observatory conVOL. XIII.-No. 1.-JANUARY, 1834. 1 structed entirely of wood, copper, and brass. The needles out of doors have only the local attraction unavoidably incident to a city location, that within doors has, of course, an irregular and more considerable attraction to affect it. The latter needle is exposed to very slight variations of temperature. These particulars are probably sufficient for the present purpose. I first saw the aurora on the evening of the tenth of July, at threequarters past nine o'clock; it appeared then as a low nebulous light, resting upon dark clouds, and interspersed by them, but was not sufficiently brilliant to make me entirely certain that it was an aurora: the test afforded by the magnetic needle shows that the phenomena began before this time, and the attention of my observer had been so far drawn towards the sudden diminution of variation at nine o'clock, that an inaccuracy in the observation was suspected, and the needle recurred to, at five minutes past nine, to verify the result before obtained. At ten o'clock, the light in the north was more distinct, extending upwards, nearly 30° from the horizon. At this time I began a systematic set of observations, which were continued until one o'clock on the morning of the 11th. By going a short distance from my dwelling to the east, I have a tolerably good view of the northeastern portion of the horizon; by going a greater distance to the west, a complete view of the north-western portion, and from the top of my dwelling a tolerable field of view to the north. At ten minutes past ten the light of the aurora was as brilliant as at the last observation, the brightest portion extending from twenty to thirty degrees east of the north point, and the diffused light extending at least 60° from the north towards the west. The upper limit of the light was a waving line declining rapidly in the eastern portion, and more slowly in the western part. The substratum of black clouds (cumulo stratus) from behind which the aurora appeared to emerge, and which at ten o'clock extended about eight degrees above the horizon, had risen, and a number of small and very black clouds (cumulus) appeared, intercepting portions of the light. At fifty minutes past ten, the waving line, forming the top of the nebulous light, had become more regular, its elevation not being, in any part, more than 12° above the horizon, its extent being about 100°. The floating hill clouds (cumulus) had elongated, diminishing in number, but increasing in size. I supposed the phenomenon to be passing off, when just before twelve o'clock commenced the most brilliant display which I remember to have seen. At twelve, the nebulous light had risen in an irregular line nearly 40° above the horizon; and to the westward of north, apparently emerging from behind the luminous cloud, were diverging beams of light, more brilliant than the body of the aurora, varying constantly in the degree of brightness, and in extent. The appearance at within a few minutes of the time mentioned, I have attempted to represent in the annexed figure, (Fig. 2, Plate I.) When I speak of the beams being diverging, I mean, of course, that they appeared to diverge; the point of divergence was below the horizon, but its position I could not, at any time, determine to my satis faction. The phenomenon resembled, in form, the beams sometimes seen in the eastern part of the sky when the sun is setting, and which appear to proceed from a point below the eastern horizon, except that these beams were of a beautiful light, apparently streaming from bebind a cloud of light. There was at this time no dark cloud visible to the north. Of the beams which I have endeavoured to describe, those nearest the north faded first, and new ones sprang up further to the west of north, much exceeding in apparent breadth those seen near the north. Small black cumulus and cumulo stratus were scattered through the cloud of light. At eight minutes past twelve, four beams were visible, the broadest being furthest to the west; the last beam died away at twenty-five minutes past twelve, vanishing, after appearing to move westward, about 60° west of north. The general light to the north was still bright, and at half past twelve the most luminous point was about 15° west of north. At one o'clock the aurora had almost ceased, there being still a feeble and diffuse light to the north. A dark cloud (cumulo stratus) occupied a portion of the horizon, having above it two smaller clouds of the same description; in the intervals between these clouds, and to the eastward of the one in the horizon, the feeble light was seen. In the following table of observations on the magnetic needle, the results which belong to the horizontal needle are referred to the same point, as a zero: to which those in May were referred; this point is not the mean of the days immediately preceding and succeeding the tenth of July, but a reference to it renders the results of the present series immediately comparable with that for May. In the observation of the dipping needle I have been perplexed by a defect of which this is not the place to give an account, but which it is necessary for me to allude to, as explaining why the observations of that needle, made prior to half-past ten P. M., are not given: those which are set down, I have referred to the first observation recorded, as a zero; as differential results, I believe them to be worthy of confidence, though not in the same degree with the corresponding ones for the horizontal needle. The differences marked + correspond to an increase of dip, and those markedto a decrease. In the column of remarks the different phases of the aurora are briefly referred to. |