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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 30% before each even hour; one at the even hour, and the third 2" 30$ 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.
January. Wednesday.. 22 Wednesday.. 20 Wednesday..19 February Friday......28 Friday......26 Friday......25 March. Wednesday..18 Wednesday..24 Wednesday.. 23 April... Wednesday.. 22 Wednesday..21 Wednesday..20 May.
Friday...... 29 Friday...... 28 Friday......27 June..
Wednesday.. 24 Wednesday.. 23 Wednesday..22 July.. Wednesday.. 22 Wednesday..21 Wednesday.. 20 August... Friday......28 Friday ......27 Friday......26 September.. Wednesday.. 23 Wednesday.. 22 Wednesday..21 October.. Wednesday..21 Wednesday..20 Wednesday..19 November. Friday......27 Friday......26 Friday ......25 December...... Wednesday..23 Wednesday..22 Wednesday..21
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.
used in observations made at other places. In our observations, the time was obtained accurately from the transit instrument and astronomical clock; and the scale which was reflected from the mirror at the southern extremity of the bar, and read off by the variation transit, easily allowed of being marked to šof a minute. The experience of the observers has satisfied them that dependence may be placed upon each separate reading within that limit of error. But other considerations make it necessary to determine the position of the magnetometer, for any assigned time, by more than a single reading. As the bar, in passing from one angle of declination to another, is maintained always in a vibratory state, it is necessary to eliminate what is due to the oscillation from what belongs to an absolute change of declination. If the arc of vibration were constant, it would be eliminated by observing the limits of excursion of the magnetised bar, and taking the mean between them. But the natural tendency of the arc of vibration is to become shorter for every new excursion, and if the arc be of considerable length, this circumstance must be taken into account. As the decrease of arc must be nearly uniform for a few vibrations, this is done by noting the limits of three successive excursions, and the mean of two means thus obtained is the true position of the bar for the middle time. Thus, if a, b, c, are the readings, i (i (a+b)+(b+c) or (a+2b+c) gives the place of the magnetic meridian for the time when b was observed. If the arc of vibration is very small, this correction will be inappreciable, and the mean of two observations will suffice. But the declination itself, meanwhile, may vary by sudden and irregular movements, and then the process of observation and reduction becomes more intricate. Facts assure us that the magnetic meridian is subject to abrupt and lawless fluctuations, as well as uniform and progressive variations. The practical mischief of these disturbed motions is diminished, by the fact that they will most probably occur during periods of unusual perturbation ; and although they must be kept in view when studying the laws of remarkable derangements of magnetic influence, their effect will be insensible in the regular and periodic changes.
A greater difficulty that affects particularly simultaneous observations is this. The precise moment of time to which the mean result corresponds may not be that for which the declination is sought; and the interpolation of the required times between the observed times is a matter of troublesome and uncertain calculation. This labour is prevented by an ingenious device of Gauss, in the way of observing. If two observations of a bar are made at an interval equal to the time of one vibration, the mean is the place for the intermediate moment. This is a proposition mathematically exact, if the change of declination can be regarded as uniform and the arc of vibration constant. It will, therefore, be practically true whenever no remarkable disturbances are apprehended, and the arc of vibration is small; or within the same limitations as the other
methods. If, now, the position of the magnetic meridian is desired for any definite moment, the first observation is made to precede this period by half the time of the bar's vibration, and the second to follow the period at the same distance. Thus, if t be the time of vibration, and T the time of mean observation, the actual observation must be made at T— t and T + £t. For greater accuracy, the final result is made to depend on several partial results, as will be seen by an illustration. The time of vibration of the Gauss Magnetometer used at Cambridge is about 5411. This is divided into as many parts as separate observations can be conveniently taken during that time. It has, therefore, been divided into 6 intervals of 911 each, and a separate observation is made at each interval. This is done during two vibrations of the needle, or 1' 48'1. By taking the mean of every two observations which have an interval of 54'', we have a partial result for the middle time, and these partial results are combined so as to give a final result for any time when the declination is required. If this time is 2h 5' the first observation is made at 2" 4' 6'', and repeated at intervals of 911 till 2h 5' 5411. An example is given from from the observations made June 26, at 0of Gottingen mean time.
24 33 42
0 9 18 27 36 45 54
The Final Mean 10.000 9.250
51 of the 7 partial 9.875 9.375 00 0 results gives 9.125 9.312
9 109.294. 9.000 9.375
18 8.750 9.375
Mean 109.294 0 0
Thus it appears that each position of the needle is determined from 13 separate observations; and as each reading is to s of a minute, the mean of all may be considered as within a smaller error of observation, and only subject to the exception, that the law of reduction is not rigorously exact when the change of declination, during 1' 4811, is not uniform. If the arc of vibration be so large as to have a sensible decrease, the effect is cancelled when the readings extend through twice the time of vibration.
On term days