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Abundant materials are furnished, by the preceding catalogue of transits, for deducing the state of the instrument, the direction of the meridian, and the true sidereal time. By means of these observations the rate of the clock is found to be

From the 10th to 15th July-1.16 From 15th to 25th July-1.41

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-1.28 Mean from 10th to 25th July.

There are three methods in common use for determining the azimuth of the transit instrument. One of them employs the successive intervals between the upper and lower passage of Polaris; another compares the transit of two circum-polar stars, whose right ascensions vary about 12 hours; while the last depends upon the transits of high and low stars, including north and south stars. When the latter method is adopted, it is advisable to select stars whose difference of declination is at least equal to 40°. There are defects in all these methods. The first supposes the rate of the clock to be uniform during the 24 hours: this may not always be the case; but on account of the slow motion of the star, a considerable error in time would make but a small difference in the azimuth of the instrument. The second process depends upon the clock for a much shorter time; but it requires four observations, and the accidental error of these may exceed that produced in the first case by the clock. Lastly, the third mode requires only two observations, and depends on the clock only for a short time: but it supposes the tabular place of the star to be accurately known. A careful determination of the meridian must be based upon a combination of the three methods; when this is done, the instrument is adjusted with great exactness. This reduction has been made out of the transits in the table, and the azimuth of the instrument, when on the meridian mark, calculated from 18 different sets of stars, selected according to the known conditions of the problem, gives, as a mean result,

0.072 west of south, being the azimuth of the south end of the transit instrument.

The three successive transits of Polaris, observed on the 14th and 15th July, give,

0.11 west of south.

The final mean is 0.091 west of south.

The octagon apartments to the west of the transit instrument contain a Gauss Magnetometer, by which changes of magnetic declination are observed. They are built of wood, with copper and zinc nails; the walls rest on wooden posts; iron, stone, and all other substances known or suspected to exert magnetic influence, having been carefully excluded from every part of the building. FD is the direction of the astronomical meridian, and D C of the mean magnetic meridian. The three circles at D are the projections of so many wooden posts, which are bound firmly together at the top, and support the marble table on which a variation transit is placed. This instrument was made by Troughton and Simms, and is used in the magnetic observations. The larger interior circle represents the table; the chord at right angles to the magnetic meridian is the projection of the scale which is read in the observations: C is a Gauss Magnetometer, which is fitted up after the style of those at the Gottingen Observatory. The three circles at C are the feet of three posts, ten feet long, which unite at the top, eight feet above the floor, and give a stable point of suspension to the needle. The rectangle, enclosed within the feet, is the box which surrounds the magnet, and protects it from currents of air. Within the box may be seen this magnetized bar in the magnetic meridian, with a mirror firmly fixed to its south end to reflect the scale at D into the tube of the variation transit. The bar is suspended by a copper wire silvered, 0.011 of an inch in diameter, 5 feet 6 inches long, and, with its fixtures, weighs about 3 pounds.

From this arrangement it follows, that as the bar varies in its position, it must carry the mirror with it. The place of the mirror determines the mark on the scale which is reflected into the centre of the telescope; so that as the reflecting surface moves, the marks of the scale that are successively seen are read off, and employed to ascertain the motions of the bar. The scale at Cambridge is so divided, that the angular motion of the needle is read directly from it without any reduction. This novel mode of observing changes of magnetic declination is a great refinement upon the old methods, and has given an accuracy to the determination of this magnetic element, which has hitherto been considered attainable only in astronomy. For a more minute description of the Gauss instruments, and the directions to be obeyed in observing with them, the reader is referred to Taylor's Scientific Memoirs, Parts V. and VI.* It is necessary to omit in this place any further details, except such as are required in order to understand the remarks which follow. As all the observations embraced in this paper were made with the

These instruments, directions, and the magnetic observatory, are fully described, with large folding plates, in vol. 3 of these Annals.-EDIT.

C

Gauss Magnetometer, I shall pass at once to them, and leave the description of the rest of the buildings, and the instruments placed in them, for the close.

The plan of magnetic observations, recommended by the Royal Society, and generally adopted at the magnetic observatories, prescribes that the declination magnetometer, and the horizontal and vertical force instruments, shall each be observed once every two hours during the 24 hours on every day of the year; that is to say, one of them is read 2m 30s before each even hour; one at the even hour, and the third 2m 30s after the even hour. One day in each month has been set apart for observations on the three instruments at shorter intervals. On these days, which are called term-days, the declination magnetometer is observed every five minutes, and the other instruments every 10 minutes, making four separate observations every 10 minutes, or 576 in the day. A short description of the instruments by which these observations are made is promised at the end of the paper. They have but recently been received and adjusted, and regular observations were made for the first time with them on the term-day of March, 1841.

The observations with the Gauss Magnetometer, which make the subject of this article, have been coincident in time with those taken with the declination magnetometer at the British stations, and are therefore comparable with them. The Report of the Royal Society on this great magnetic adventure, provides that the observations shall be made as closely as the nature of the instruments, with all the recent refinements of mechanical skill, added to the ingenious artifices of the observer, will permit; and the time is to be carefully noted, after having been determined by the assistance of astronomical instruments, so as to make the observations at the different stations, in a practical sense, simultaneous. Observations, according to this rigid system, were originally expected to extend over three years. When the materials, thus patiently gathered from so many remote sources, shall be collected, they will pass into the hands of competent persons, to furnish the elements for a complete theory of the earth's magnetism; and any attempt to pronounce, at this time, positively, on this great subject, would justly be regarded as premature. But it is equally certain, that there are secondary questions, of considerable intrinsic importance, which may be discussed now as well as at any time, the settlement of which will be facilitated by an occasional publication of portions of the regular magnetic observations. Much light may be thrown upon the general subject by a comparison of partial results; and parts of a well-concerted plan, to which no present objection appears, may prove useless in fact, and be either remodelled or superseded by other observations, which experience has shown will serve a better purpose. Moreover, the subject of terrestrial magnetism deeply engages the attention of the scientific world, and many, not actively engaged in the research, will be anxious to understand the progress of a scheme which promises

to shed light upon this complex problem. In this great country, it is highly important that something should be done to awaken the attention of individuals, and of the American nation, and to bespeak their bounty in favour of a scientific enterprise, which has received the highest patronage of European and other governments. The commercial prosperity, no less than the interests of pure science, will be affected by the spirit and liberality with which this bold project is sustained and conducted. It is hoped that the present publication may not prove useless for some of these purposes. A writer in the "London Quarterly Review," after speaking of the materials that will be collected at head-quarters, by three years' contribution, proceeds thus:-" Voluminous beyond all former precedent as the mass of data thus accumulated must of necessity be, we trust the whole will be printed (each nation and each department, of course, providing for the publication of its own). No consideration of economy should be allowed to interfere with the performance of this necessary duty, without which we look upon all that shall be done as virtually thrown away. Highly as we respect the illustrious body above mentioned, and applaud their selection of the individual into whose hands the results will in the first instance pass, yet their full, fair, and effectual discussion, can be secured by no other means than by inviting to it the collective reason of the age, and of all succeeding ones, and affording every one who may think proper to engage in the task, now or hereafter, ample opportunity to do so.

It should be stated, for the information of those who have not watched the progress of the recent investigation into the laws of the earth's magnetism, that it originated with Baron Humboldt, and Professor Gauss, of Gottingen. In 1828, Baron Humboldt laid the foundation of the German Magnetic Association, by erecting a magnetic observatory at Berlin, which was imitated in various parts of Germany and Russia. In 1833, the attention of Gauss was called to the theoretical consideration of the earth's magnetism, but he found himself soon arrested by the want of accurate and extensive data. He instituted accordingly a magnetic observatory at Gottingen, and, having furnished it with new instruments, began observing in March, 1834. Co-operation was sought and obtained in various parts of the continent; a more intimate alliance was formed between the several points of observation; and, after various modifications in the time and mode of observing, the amount of which was, to make the observations more minute, and the number of terms smaller, one day in the months of February, May, August, and November, was selected, in which observations were to be made at every five minutes during the 24 hours. These are the present German magnetic term-days. In this advanced stage of the research, Humboldt addressed a letter to the President and Council of the Royal Society, soliciting the countenance and support of the British nation; and this appeal, after having been met in the most prompt * London Quarterly Review, No. cxxxi, page 303.

and generous manner at home, was sent abroad to various Academies, and men of science in this country. The extent and important magnetic position of America were insisted upon in the appeal sent out to it, and it is to be wished that the example set by the magnetic observatory at Philadelphia, under the care of Professor Bache, and by the American Academy of Arts and Sciences in Boston, who have supplied the Cambridge Observatory with the requisite instruments, will be a motive to our government, and to individuals, to take measures in this matter worthy of their wealth and energy, as well as the eminent local advantages which they enjoy in regard to this scientific investigation.

The following table, from page 38 of the Report of the Royal Society, is published here for the information of those who have not seen the original.

Days of Commencement of the Terms of Simultaneous Observation, during the years 1840, 1841, and 1842.

Month.

January.

February.

March.

April..

....

May..
June..

July..

August...

September...

October...

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Wednesday..22 Wednesday..20 Wednesday..19 Friday......28 Friday......26 Friday......25 Wednesday..18 Wednesday..24 Wednesday..23 Wednesday..22 Wednesday..21 Wednesday..20 Friday......29 Friday......28 Friday......27 Wednesday..24 Wednesday..23 Wednesday..22 Wednesday..22 Wednesday..21 Wednesday..20 Friday......28 Friday......27 Friday......26 Wednesday..23 Wednesday..22 Wednesday..21 Wednesday..21 Wednesday..20 Wednesday..19 November... Friday......27 Friday......26 Friday......25 December...... Wednesday..23 Wednesday.. 22 Wednesday..21

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The observations on the term-days, with the Gauss Magnetometer, were begun at the Cambridge University, in March, 1840, and no one has been omitted since that time.* The curves for only two of these days, the 29th of May and the 21st of October, are published; but the numbers which express the mean results for every five minutes during the 24 hours, on all the term-days, are placed in Table I, at the end of the paper. This mode is preferable in some respects to publishing the curves, as it enables any one who intends to make a comparison, to draw a curve for Cambridge, on the scale

As the Report of the Royal Society, containing the details of their plan, had not been received when the observations commenced, two of the term-days were incorrectly taken, and the observations in April, besides being undertaken on the wrong day, were so imperfect, that they have been left out of the account entirely. Instead of March 18, March 27, and instead of July 22, July 24 were observed at Cambridge. This must be considered in the comparison of these days with observations made elsewhere.

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