Besides the fluctuation to which the needle is incident in this respect, another peculiar propensity, termed the dip, affects the regularity of its other motions very materially. When it has been nicely balanced on a horizontal axis, it is found to decline gradually from its horizontal position, and to settle at a very considerable angle with the plane of the horizon: which angle, like that of the variation, differs constantly at different places of the earth's surface.

It has moreover been found to have a daily periodical motion, or shifting backwards and forwards: that is, it veers during the forenoon a little towards the west, and returns in the afternoon towards the east ; and the quantity of this daily deviation is not always the same, but differs according to the temperature; being greater in summer than in winter, but less in the tropical than in milder climates.

As every magnetic needle is subject to these aberrations, to whatsoever part of the world it may be conveyed, they may be termed universal affections: but it has other irregularities of a less general character, though perhaps not less important and perplexing; one of which only we shall here notice, as being immediately connected with the experiments and results now under consideration.

It has been already stated that, of all substances, the magnet is most susceptible of the influence of iron; and, as considerable quantities of this metal are distributed throughout every fabric of more than ordinary dimensions, it is almost impossible for a magnet to be practically situated and yet be out of the reach of such local attractions. Now the greatest purity of these magnetic laws is requisite on ship-board; and, as vast quantities of iron are placed in and about every part of a vessel, the needle is always found to be materially impressed and disturbed by the combination of this confused local influence: so that, its true bearing being thus, in all situations, considerably changed, the information meant to be derived from it in critical cases, when it would be most desirable, is consequently rendered dubitable. To the error arising out of this local attraction, the attention of Mr. Barlow has been most expressly directed; and for this evil we conceive that he has been fortunate enough to propose an efficient remedy, or at least a valuable modification of the effect.

Before we fully enter on a report of the labours and discoveries of Mr. Barlow, it will not be amiss to afford a preliminary view of the opinions of those who have been most attentive to the subject and consequences of local attraction. In our lxxxivth volume, p. 301., will be found some comments on a tract relative to this question. We then took an opportunity of noticing

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ticing that "Mr. Wales, who accompanied Captain Cook in his several voyages for the purpose of recording astronomical observations, was the first who seems to have discovered the effect on the needle of that attraction which was produced by the matter within the ship;" and we inserted an extract from the introduction to some astronomical tables, in which Mr. Wales comprized his remarks concerning this local influence, to which we beg to refer such of our readers as may desire to know under what circumstances it was first observed. From the time of its discovery by Mr. Wales, nothing important appears to have been published relative to local attraction until Capt. Flinders, after his voyage to Terra Australis in 1801 and 1802, renewed the subject. He had paid during his voyage much attention to it, and seems to have obtained the first correct ideas with regard to its being connected in some measure with the phænomenon termed the dip. Capt. Flinders, moreover, deduced from his own observations a rule for estimating the aberration consequent on local attraction: but this rule has since proved inadequate to effect the purpose which was intended. In the article above quoted will also be found, besides some farther information relative to the observations of Capt. Flinders, the account of " An Essay on the Variation of the Compass, shewing how far it may be influenced by the Direction of the Ship's Head; with an Exposition of the Dangers arising to Navigators from not allowing for this Change of Variation; interspersed with practical Observations and Remarks. By W. Bain, Master in the Royal Navy."

We have since had an opportunity of learning the result of a series of recent observations made by Capt. Sabine, in Capt. Ross's late voyage of discovery to Hudson's Bay; it being previously ordered that the effect of local attraction on the needle was to be one of the phænomena to which his particular attention should be directed. (See M. R. vol. xci. p.184.) As we have already had occasion to insert the observations of Capt. Sabine in our notice of the communications made to the Royal Society, it is not necessary now to do more than refer to the article as above; which also contains, besides his remarks on the Irregularities of the Compass Needles of the Ships Isabella and Alexander, caused by the Attraction of the Iron contained in the Ships," his "Observations on the Dip and Variation of the Magnetic Needles, and on the intensity of the Magnetic Force;" together with a communication made also to the Royal Society by W. Scoresby, junior, Esq. "On the Anomaly in the Variation of the Magnetic Needle as observed on Ship-board." In perusing the pages which contain our review of these papers, the reader will be enabled

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to derive, from the account of a variety of practical observations made with the most perfect apparatus, and with unquestionable skill, a proper knowlege of the state in which the inquiry respecting the influence of local attraction stood immediately before the important discovery of Mr. Barlow was announced; and we cannot refrain, at this point of our remarks, from congratulating that gentleman on his having so completely anticipated the strong injunctions urged by us, when the subject then came under our consideration, on the necessity of pursuing with still greater determination this interesting inquiry, and recommending it to the attention of our mathematicians and men of science.

As the whole of the experiments conducted by Mr. Barlow were throughout similar to one another, except that the iron balls which he employed were sometimes shells and in some instances solid, sometimes small and sometimes large, it will only be necessary to state the nature of the experiment. Having provided a proper table, and on it described a number of concentric circles, the circumferences of which he divided into degrees, &c.; and having placed a ball of iron precisely in the common centre of the several circles, he proceeded to move the compass carefully from point to point about each circumference, strictly observing at every position the effect produced on the needle: also elevating and depressing the compass above and below the ball, perpendicular to each of the circumferences; and accurately remarking the position of the pivot of the compass with respect to the centre of the ball. After having repeated these experi→ ments at different distances from the ball, and confirmed his observations by a variety of the most delicate instruments, Mr. B. at length happily succeeded in deducing this positive conclusion; that, in a certain plane, the iron ball had no influence on the needle; and that this plane is exactly, or very nearly, perpendicular to the direction of the dipping needle: the dip in this latitude being about 70° below the plane of the horizon, and the angle of the plane of no action being about 20° above the plane of the horizon; the angle of the dip being consequently the complement of the angle of the plane of no action. Thus resulted, from the first series, of experiments, a new and valuable fact relative to the mysterious law of magnetic attraction. It ought, however, to be here mentioned, in justice to Capt. Flinders, that, though he never positively arrived at this important fact, yet those observations must have been very acutely made which enabled him to infer the relative direction of the dip and the angle of local attraction; and which furnished him with a rule, partly

correct, for counteracting the consequence of errors in computation resulting from this particular discrepancy.

The next object was to ascertain how far the discovery would contribute to supply the necessary rules for computing the effect of local attraction in any latitude, or longitude; and also with respect to the distance or quantity of the operating mass. In determining his several formulæ and deducing his rules, Mr. Barlow has not only displayed much ingenuity, but has evinced an eminent portion of mathematical knowlege.

The several laws determined are these:

For the Latitude: - The tangent of the angle of deviation is proportional to the rectangle of the sine and cosine af latitude; or to the sine of double latitude, the longitude being zero.

For the Longitude: - The tangent of the angle of deviation is proportional to the cosine of longitude, the latitude being

constant.

General Rule, the latitude and longitude both changing: The tangent of the deviation is proportional to the rectangle of the cosine of the longitude, and the sine of the double latitude.

For the Distance of the Mass: The tangents of the angles of deviations are reciprocally proportional to the cubes of the distances.

For the Mass:- The tangents of the angles of deviations are proportional to the cubes of the diameters, all other things being the same.

No rational doubt can be entertained of the efficiency of any rule, the datum of which is fact determined by experiment, and when the operations of the rule are afterward corroborated by a series of experiments, as in each of the foregoing instances; where the several results of each particular rule, corresponding to the course of experiments, are given in tables; and it is not a little curious to witness the close approximation that appears in almost every instance between the experiment and the computation.

The investigation of the law of attraction, as it regards the mass, has tended to the disclosure of another very important secret in the law of magnetic attraction.

The cubes of the diameters being proportional to the masses, the obvious conclusion,' says Mr. Barlow, seemed to be that the tangents of the deviation were also proportional to the masses; and such, in fact, was the conclusion I had drawn, when I fortunately made trial of a ten-inch shell whose weight was 961b., or just three-fourths of that of the last solid ball of the same dimensions, and I was not a little surprised to find that I could observe no difference whatever between these results and the former. I then determined

determined on a regular course of experiments with the shell, at the same distances, &c, as I had adopted with the ball; and having completed them I found, on a comparison of the results, that they tallied with each other throughout. In fact, it appeared that the power of attraction resided wholly on the surface, and was independent of the mass.

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Being, however, unwilling to leave any thing doubtful respecting a result which appeared so extremely novel and unexpected, I tried two other ten-inch shells, lest there should have been any thing peculiar in the one referred to above; I then employed other shells of different diameters and thicknesses, the whole of which still indicated the same law, viz. that the tangents of the deviation are proportional to the cubes of the diameters, or as the power of the surface, whatever may be the weight and thickness.

This law, however, I have since found to have its limits; for having procured a ten-inch shell of tin, and another of iron, the weight of the former being 43 oz. and of the latter 45 oz., I found the power not so great as in the solid ball of iron, although the approximation was very near, considering the great disparity in the weights; the iron shell producing deviations which were to those of the solid ball as two to three nearly. Now the thickness of the iron being here at a medium about one-thirtieth of an inch, the conclusion which we may draw from this fact appears to be that the magnetic fluid requires a certain thickness of metal exceeding one-thirtieth of an inch in order effectually to develope itself, and to act with its maximum of effect.' (P. 43—45.)

This conclusion is afterward confirmed by a series of experiments, and has certainly unfolded a new and interesting secret in this curious branch of physics.

In prosecuting some of his experiments with a very fine instrument fitted up in a brass box, Mr. B. found himself greatly perplexed with results contradictory to those which had been before obtained; and, after having given a description of this perfect instrument, (as it had been esteemed,) he says:

I have been particular in describing this instrument, not because I made much use of it in my experiments, but because I found a defect in it which may probably more commonly appertain to compasses of this description than is usually imagined, and which, I conceive, is important to be made public. Having, immediately after my apparatus was erected, repeated with the above instrument a few of my former experiments, I found myself considerably perplexed with certain anomalies and irregularities which I could not account for on any principle, till at length it occurred to me, that they were precisely what would take place, if any part of the brass box itself had become magnetic, and on trial I found this actually to be the case; for, on removing one of the pieces of brass attached to the box, for the purpose of setting the instrument and fixing the sights, I found it to be strongly magnetic, sufficiently