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and that from 1810 to 1840 it increased at the mean annual rate of 3'.6. It is probable that the change in direction took place between 1793 and 1810 at Cambridge; whatever was the cause, it does not appear to have affected instantaneously remote places. Dr. Bowditch, who made the observations at Salem in 1805, 1808, and 1810, supposed that the first two were smaller than they ought to be, on account of instrumental errors; but I think that this was not the whole cause. The change in direction, if it had not already happened in 1810, came soon after. Now, on the assumption that it was after 1810, the mean annual rate of decrease at Salem (considering the observations at Beverly as comparable with those made at Salem) would be only 1'.3. This is smaller than the decrease appears to have been on the average. I incline to think, therefore, that the minimum declination really occurred during the period of Dr. Bowditch's observations, and that the differences which he attributed wholly to defects in the instruments, were partly caused by this very circumstance. The value of the variation at Beverly in 1781 was obtained from the mean of 7 partial means, which did not differ more than 6 minutes from each other. The variation at Salem in 1810 was the mean of 5125 observations. Confidence may accordingly be placed in both of these values, and consequently in the mean annual decrease of 1'.3 that results from them. Again, the variation at Boston in 1793 is the mean of 1644 observations. The variation at Cambridge in 1782 is deduced from frequent observations at different hours on 127 days of that year. Allowing a reasonable error to beset the value at 1708, we may suppose that the number of years from which the rate of annual decrease is derived secures it from any large error. The same reliance may reasonably be placed in the determination of the rate of increase. The fact, then, is very remarkable, that since the passage from a direct to a retrograde motion the rate of the annual change of variation has so materially altered. Assuming that the mean annual rate of decrease was 1'.8, the time of the change interpolated into the observations would be 1807; and this, in the absence of better authority, may be regarded as its date for Cambridge.

Another element of the earth's magnetism, on which some attention has been bestowed at Cambridge, is the dip. All the methods of observing the dip are extremely defective, and do not admit of so great a degree of accuracy as the declination instruments. The dipping-needles of Gambey, and Troughton and Simms, are preferable to any other direct method of measuring absolute dip. But two of the best dipping-needles may vary 15' and more, in the determination of this element for the same time. This difficulty

* In most cases of this kind, the rate varies about the times of maximum and minimum as the time, reckoned from these points respectively; so that the value of the element is in proportion to the square of this time. But in the present instance the simple supposition we have made in the text conforms best to the observations.

suggested to Gauss the idea of expressing the dip as a function of the horizontal and vertical components of the magnetic intensity. This led to the invention of his bifilar magnetometer, and the horizontal force magnetometer of Lloyd. Professor Lloyd has added to these another extremely delicate instrument, by which variations of the vertical force may be observed. Consequently, with his two instruments, the changes of dip may be calculated to a close approximation, though the absolute dip remains unaffected by all the improvements. In addition to the usual corrections applied to observations made with the dipping-needle, the precaution should be taken of observing the inclination in different azimuths, and deducing the true dip from every set at right angles to each other. Mr. Fox has invented a dipping-needle deflector, which gives the dip and intensity by a statical principal, and Mr. Lloyd has applied the same principle to the simultaneous determination of the dip and intensity. Observations with the vertical force magnetometer commenced with us on the term-day of March, 1841; but they do not enter into the plan of the present paper, and any further notice of them, or of the instruments, is deferred till we come to the description of the new magnetic observatory. The dip has been observed directly at Cambridge and the vicinity, during the last four years, at irregular intervals. The dipping-needles used were of the best construction, one of them made by Gambey, and the other, now in the possession of Major Graham, obtained from Troughton and Simms. The following table gives the results, with some old observations on the same element :

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Supposing that the inclination has been on the increase since 1782, its mean annual rate would be equal to 4.'5. It is thought that the inclination is decreasing at present in the United States, as it has long been in Great Britain and on the continent. Professor Loomis assigns the rate for this country at 1.'8 yearly. If this be so, we

* This observation was made by Professor Loomis.

observe the same great difference in the rate of increase and decrease of the inclination as of the declination. In either case, it is equally inexplicable. The want is now felt of systematic observations of the dip, in order to eliminate regular and irregular changes of this element, and obtain a true mean for a given period. The general scheme of magnetic observations has not failed to provide for this, as we shall see further on. So far as the subject has been investigated, the dip does not appear to pass through such a uniform succession of positions daily as the declination. Its value often changes suddenly, and to the amount of a degree and a half, although, according to Kupffer, the regular daily range does not exceed five minutes. As the various observations on the dip, at Cambridge and Dorchester, were made with the same needles, the range of 21' in the values cannot be charged to the needles, and it is too great to come under the daily or annual variation. Observations made elsewhere shew that such irregular disturbances occasionally derange the dip. Moreover, the mean value, derived from frequent partial results obtained on different days and different hours of the same day, is still exposed to what may be called a constant error of the needle, and which is not eluded by any of the different reversals. Thus, out of 8 needles used by Captain James Ross, in London, 2 differed 41' in the values which they gave for the dip; although from 640 to 1,000 readings were made with each. We subjoin the following table, drawn up by Quetelet, which contains the value of the annual diminution of the magnetic inclination at various places :

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* Nouveaux Mémoires de l'Académie Royale des Sciences et Belles Lettres de Bruxelles. Tome XII. 1839.

(To be completed in the next Number.

MAGNETIC PHENOMENA.

[For Declination read Variation.]

On the 25th of September of the present year, a most extraordinary disturbance of the magnetic instruments was observed at the Magnetic Observatory attached to the Royal Observatory of Greenwich. The disturbed state of all the instruments attracted the attention of Mr. Glaisher (chief assistant in the magnetic department), at an early

hour of that day, and he immediately commenced a series of observations on all the instruments, at short intervals. After a time, the disturbance became so small that the extraordinary observations were discontinued; but it again increased, and observations were again immediately made with all the instruments, and repeated as rapidly as it was possible for one observer to do so. But for the promptitude and judgment displayed by Mr. Glaisher on this occasion, the record of this disturbance, unprecedented in this magnetic latitude, would probably have been lost. The following statement will give an idea of the magnitude of the disturbance :-That within eight minutes of time the declination needle changed its position more than 21 (having passed in both directions the range of the observing telescope, which includes that angle); that the vertical force was increased by more than 1-40th of its whole value, the instrument having then reached the extremity of its range; and that the horizontal force was increased about 1-30th of its whole value.

The following more detailed account is taken principally from the abstract furnished by Mr. Glaisher. It is to be remarked, that the time throughout is Gottingen mean solar time, civil reckoning; that the readings of the theodolite for the declination needle increase as the north end of the needle moves towards the east, and that the reading for the astronomical meridian is 269° 51′ 45′′ nearly; that the increasing readings of the vertical force magnetometer imply increasing vertical force, one division being equal to 0.000471 of the whole vertical force; and that the increasing readings of the horizontal force magnetometer imply increasing horizontal force, one division being equal to 0·002214 of the whole horizontal force.

Early in the morning of Sept. 25th, the needles were in an agitated state. During the appearance of an aurora, additional observations were taken, and the declination needle in less than three hours traversed an arc of 34'. After this, the needles were in a tolerably quiet state, and extraordinary observations were discontinued.

The observations at 10 A.M., showing a change of 17′ of arc from the previous reading, at 8h A.M. extra observations were again resumed, and continued till 11 A.M.; nothing remarkable appearing during this time, they were again discontinued.

At 2 P.M., Gottingen mean time, it was evident that all the needles were affected by some unusual change of disturbance, and incessant observations were at once commenced, the instruments being observed successively, as quickly as one observer could take them. At 3 53 Mr. Hind joined, and subsequently Messrs. Dunkin and Paul. From this time to the discontinuance of the observations two persons were constantly engaged, one taking the observations with the vertical force magnetometer, the other those of the declination needle and of the horizontal force magnetometer. About 2h 40m P.M., the motion of the needles became very peculiar ; the vibrations of the declination needle, and of the vertical force magnetometer, being quite destroyed; and those of the horizontal

force magnetometer being reduced to a continuous succession of jerks or starts. It will perhaps contribute to clearness if the motions of the three instruments are now separately described.

Declination Needle.

Every change in this instrument from one position to another was by sudden impulses; after each of those it was stationary, for intervals varying from 5 to 20'; then it was forced again to another position, and was again stationary without the slightest motion; and so on successively.

At 3h 30m the changes from one position to another became very decided, but the motion still partook of the same character; absolute rest and jerking motion followed alternately; in 2 or 3′ the needle frequently moved through many minutes of arc, and then suddenly became stationary again.

*

At 3h 36 20 P. M., a bold sweep carried the north end of the needle towards the west 56′ of arc, in one minute of time; at 3 36m 20" the theodolite read 245° 22′-; and for two minutes after this time the cross carried by the needle was out of range of the telescope, being in a position in which, if the telescope had been exactly directed to it, the theodolite would have read less than 245° 22′. At 3 40" 20" P. M., Gottingen mean time, the needle had moved back 25′; at 3h 43m 50' the theodolite read 246° 19′; at 3h 44TM 20′ the needle had moved so far (north end towards the east) that the theodolite reading was 247° 38'+, and the cross carried by the needle was out of range of the telescope on the opposite side.

This extraordinary magnetic stride of more than 24° was traversed in about 8", the extremes being only separated by that interval of time.

At both extremities the needle was without swing, all momentum from such a wide sweep being quite destroyed. After remaining at this place for more than two minutes, the needle slowly returned, and at 3h 48m 54* the theodolite read 246° 27′; at 3h 53m 21' it read 246° 37'. Within 5' after this, another bold sweep carried the cross out of the range of the telescope; when found, at 3h 54m 40', the theodolite read 245° 31′+; instantaneously it rushed back, with a violently agitated motion, across and out of the field on the other side; the circle-reading, at 3h 55 0', was 246° 43′+, and therefore this arc of 1° 12'+ was passed over in 20′ of time.

* This arose from the great inclination of the axis of the collimator (carried by the magnet) to the magnetic meridian; in consequence of which the pencil of rays passing through the object-glass of the collimator was thrown entirely on one side of the object-glass of the theodolite. In ordinary cases, the change of angle has been sufficiently slow to allow the observer to shift the suspension of the magnet; but the rapid changes on September the 25th did not allow the observer to leave his place. Observations with the mirror in Gauss's manner are free from this inconvenience.

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