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their remarkable frequency and magnitude. We annex a brief history of each month in this respect.
1840. March 27-8. No perturbations of importance; only two unusual excursions, one at 2h 45', A. M. Gott. M. T. (March 28), and the other at 4" 15', A. M.
April. The observations of this month are very defective.
May 29-30. Term-day. Irregular disturbances of large amount, from 11 o'clock, P. M. Gott. M. T. (May 29) to 125 of May 30. The whole
sweep of the instrument through the day is 57',2; and once, between 36 50' A. M. and 4" 10, the declination changes 47', 2 in 11", or four times the average daily swing from maximum to minimum. The other days observed in May were not distinguished above the average by perturbations.
June. The ten days observed in this month were all unusually quiet.
July 24-5. Between 6 and 8 o'clock, P. M. Gott. M.T. (July 25) the arc of vibration of the magnetometer amounted to 20', so as to require the substitution of its extreme limits instead of the 12 readings at intervals of 10 seconds. But this large movement was not accompanied by any considerable change of absolute declination, and the whole magnetic day was undisturbed.
August. The Term-day of Aug. 28-9 was greatly deranged from 104 40' P. M. Gott. M.T. to 8" A. M., and small perturbations were experienced for four or five hours after this time. The whole range of the magnetic declination amounted to 61'. Once between 61 40' and 7" 20' A. m. the change of declination exceeded 43' in 27 minutes. The other days observed in August were still.
September 21-2. The sweep during this day was 45'.5. The disturbances on the days observed in this month have been already discussed at length. They were distinguished more by number than extent of arc.
October. The October curves were not entirely free from disturbances, though they are all comprehended, in their widest excursions, in a zone of 22' in breadth.
November. The three days of this month have some perturbations, but none deserving especial notice.
December. Two of the curves observed in this month were disturbed considerably; yet the whole range does not surpass 28'.5.
1841. January. The curves observed in this month were generally regular. On the 26th of January, between 45 and 5" A.M. Gott. M. T., a small disturbance was felt amounting to 17'.
February. General perturbations spread over the three days of this month, particularly observed at night. Those of greatest extent occurred on the term day, February 26-7, amounting in one case to 16' of arc in 15 minutes of time. In addition to these
extracts from the records of the different months, Table II, contains a column showing the extremes of the magnetometer every day when complete observations were made.
The theory of the aurora borealis, which has of late years found most favour with men of science, supposes it to have some connexion with electricity and magnetism. It is important to investigate this subject further, and see whether there be any and what relation between this brilliant appearance of the heavens and the derangements of the magnetic declination. For this purpose a careful record has been kept of all the auroral appearances that have been noticed at the Cambridge Observatory; some of the most remarkable presented themselves at times when the regular observations on the declination magnetometer were in progress, and pains have been taken, whenever it was practicable, to watch the instrument on all other occasions when the heavens gave signs of preparation for such an exhibition. Annexed is a list of those which were displayed on a grand scale.
April 24-5. Slight aurora.
May 28-9. Remarkable aurora. An arch was formed, at 25 39', A. M. Gott. M. T., running, as nearly as could be ascertained, at right angles to the magnetic meridian. A crown began to form at 4" 24'. Its position was referred to a Cor. Borealis, which was then on the meridian. As it was nearly at the same altitude of 74° 52', and to the west of the meridian, it could not have been far from the magnetic pole. Shortly after this the arch was broken up, and the northern sky covered with pulsations of light.
May 29-30. Brilliant aurora. The auroral arch was first seen at 25 32' A. M. Gott. M. T., extending from a point nearly east, to within a few degrees of the western horizon. The light was intense. The apex of the arch was situated 28° at first, and at 25 42', 30° south of the zenith. After this time, the light became broken and scattered, flying from east to west.
This arch was entirely detached from the main body of the aurora, and resembled a streamer. In the north there was a diffused light, but very bright; and swift flashes towards the zenith. At 3h 59' a large meteor was seen in the north, 20° high, descending towards the north-west.
June 26-7. About 2h 36' A.M. Gott. M. T., an aurora was seen at the north, of a white diffused light. At 6h 20', the aurora assumed a dull appearance, with dark wane intermixed. At 7" A.M. the aurora became more active, and some streamers were The needle was slightly affected at this time.
July 4-5. Between 5' 28' and 5" 34' Gott. M. T., bright diffused northern lights; occasionally long streamers; wane clouds near the northern horizon ; magnetometer quiet.
July 29-30. At 2 45' A. M. Gott. M.T. (July 30), an auroral arch was formed 7° above the horizon, and very still. At 9, 40' it began its motion up towards the zenith ; rose to the altitude of
30°. At 3h 54' the aurora had ceased. The light was dull during the whole time.
August 19-20. A steady auroral arch was observed. It was double, and the altitude of its apex at 3h 54' A.M. Gott. M. T. (August 20) was 7° or 8° ; its colour was dull white.
This aurora continued till 4", and at 4" 11' the northern lights had entirely disappeared.
August 28-9. An auroral arch appeared running from east to west, of intense brightness and diffused, but without streamers. Apex nearly on the meridian, and altitude 45° at 2h 30' A. M. Gott.
At 3" 20' streamers shot up 60° from the horizon; the altitude of its highest part was about 76° 43', as found from its place among the stars.
October 22-3. Between 1 and 25 A. M. Gott. M. T., an aurora of a steady blue light was first perceived; it afterwards became brighter and whiter; the altitude was 3° ; wane clouds below. At 56 20', the aurora was low.
November 30. At 3' 24' A. M. Gott. M. T., an aurora was seen of white diffused light. No regular arch was formed. The magnetometer was quiet.
It appears, from this abstract of the records, that the days most distinguished for auroral appearances, are just those on which the declination of the magnetic meridian experienced the most extraordinary derangement. This was the case on May 29-30, and August 28-9. Unfortunately the magnetometer was not watched on the night of May 28-9. By referring to Plate IV, it will be seen that the declination instrument was subject to more than ordinary influences on the 22nd of October, between 0 m. and 6" A. M., Gott. M.T. The observers on the remarkable days of May and August describe the motions of the magnetometer as peculiar in the highest degree. It was often checked in the midst of its vibration, and suddenly forced back in the opposite direction ; and this took place with such frequency at certain seasons, as to give to the motion the appearance of jerks or sharp twitches. No correspondence was noticed between the time of maximum magnetic disturbance and the formation of the auroral crown. But it was sometimes supposed, from successful comparisons in the phases of the different phenomena, that the instrument gave intimation, by some strange motion, of the most signal changes in the aurora. The display of May and August was as fine as any that has been witnessed for several years, and we should not omit to state, that on these occasions the declination magnetometer at Cambridge made the boldest sweep of the scale. As both these days happened to be term days, the opportunity was improved at other magnetic observatories of watching the coincidence between the auroral appearances and the perturbations, and the report is generally uniform from all. Plate II, which represents the May-term diurnal curve of declination, offers a specimen of the extraordinary disturbances to which we refer,
and the time of them may be compared with the phases of the aurora, which are contained in the record for that day. The excursion at g was so great, that it was found necessary to curtail it on the plate; but the extent will be readily seen from remarking that it reached to 71.4 on the scale, 47'.2 of which were traversed in 11 minutes of time. An aurora was seen on the same night at Philadelphia, New Haven, and at Toronto, U.C. A description of its appearance at New Haven may be seen in Silliman's Journal, No. 1, vol. xxxix.* Where facilities existed for making the observations, it was discovered to be accomplished with similar effects upon the magnetic declination as were felt at Cambridge. The magnetometer at Philadelphia experienced great derangements, although the limits were less, not exceeding 55'.8. The influence which an aurora exerts upon the earth's magnetism reaches as far and wide as the appearance itself; and probably the intensity of the effect is proportional to the brilliancy of the display. The greatest disturbance of the magnetometer at Philadelphia was, as at Cambridge, between 4h and 5" A. M., Gott. M. T. The deflection of the instrument at Cambridge amounted to about 57 minutes, and the extremes were separated by little more than 2 hours. Lieutenant Riddell informs us that at Toronto the arc traversed was 1° 59', which was never equalled, and approached but once on a similar occasion. We also learn from him that an aurora was noticed at Greenwich, Great Britain, on the same day; but he adds, that the disturbances there and at Toronto were very different.
Such full information is not possessed in regard to the aurora of August 28-9. It is evident, from the observations, that the magnetometer at Cambridge was more affected on that day than ever before, the whole change of declination amounting to 61'. At Toronto, where the aurora was also seen, the disturbances were equally surprising, and produced an oscillation of 1° 33' in declination. The greatest amount of derangement at Cambridge was as follows :At 123' A.M., Gott. M.T., the reading of the scale was 111.9
Range of 52'.4 East in 1" 10'. “ 2 35'
Range of 52.9 West in 1 hour. 3 35'
111.4 Again, at 5h 20' A.M., Gott. M.T., the reading of the scale was 108.5
Range of 47'.6 East in 25 minutes. 5 h 45'
156.1 Range of 52'.4 West in 1 hour. 6h 45'
During the first of these periods, the aurora reached its culmination of splendour ; between 56 and 76 it was faint and near the
See also the Journal of the Franklin Institute for June, 1840, which contains some observations made upon it at Southwick, Mass.
horizon. It does not appear, from an examination of the May or August term-day, that the maximum agitation of the magnetometer coincides in time with the greatest brilliancy of the heavens. In May, it had not accumulated its action when the aurora began to decline ; and in August, although it accompanied the display, it continued with undiminished energy one or two hours after that had passed away, The most rapid motion of the bar was from 55 20' to 56 45', being equal to 47'6. in 25 minutes. This is nothing strange; but might be expected from the time which all the forces of nature consume in communicating themselves to bodies, and penetrating large masses so as to overcome their inertia. More exact and frequent observations will doubtless conduct to a better knowledge of a connexion which is now so undeniable, and yet so imperfectly understood. If observers are careful to note the times at which the chief phases of the aurora are witnessed, and its position among the stars, and, where they have the opportunity, the simultaneous variations of the magnetometer, we may not despair of elucidating these two classes of intricate and interlaced facts—the aurora and the irregular perturbations of the magnetic meridian. It must not be inferred that other causes do not exist, in co-operation with the auroral phenomena, to derange violently the earth's magnetism. According to Ampere's theory of currents, a large fund of such derangements must be deposited under the crust of the earth. The equilibrium, although permanently stable, must be subject to constant fluctuations. Theory supplies the reason, and observation asserts the fact. Many of the small daily derangements have no apparent relation to the aurora ; and in regard to the magnificent strides of the magnetometer, it cannot be told which is cause and which is effect. If further search shall prove that an aurora never fails to attend a great disturbance, we may conclude that the aurora itself is seldom displayed in the daytime ; for the remarkable changes of declination almost always begin during the night, and seldom continue into the next day. If, however, an unseasonable aurora should occasionally arise, we may be able to perceive indications of its presence from the magnetic perturbations, although its light were eclipsed by the brightness of the sun.
So far we have attended to relative only, and not to absolute declinations. The former are sufficient when the object is to find the times of maxima and minima, the daily range, and the diurnal curve. But it sometimes becomes necessary to know the absolute declination, so that the process will now be described of referring any reading of the scale to its absolute value. It is clear that if the absolute value of one reading can be ascertained, that of all the rest is known at once. It is convenient to have the absolute declination always referred to the same number of the scale; we will suppose this number to be, therefore, 100. To find, then, the absolute declination corresponding to 100 of the scale, we proceed thus :--The variation-transit, with which the Gauss magnetometer is observed,