THE

Boston Journal

OF

PHILOSOPHY AND THE ARTS.

ART. XXIII.-A popular View of Mr BARLOW's Magnetical Experiments and Discoveries, particularly as they have been rendered applicable to the Correction of the Local Attraction of Vessels. [Edin. Philos. Jour.]

SEVERAL detached notices of Mr Barlow's experiments and results, have been given in some of the preceding volumes of this Journal; but the great importance of them to nautical science, has been lately so strikingly demonstrated by a Report of the Author's, addressed to the Admiralty, detailing a series of experiments made, by order of that Board, in his Majesty's vessels in various parts of the globe, that they now possess a new and more general interest; and we feel assured, that a connected and popular view of the whole subject. will be acceptable to our readers in every part of Europe.

Of the numerous interesting facts with which philosophy has been from time to time enriched, by far the greater number may be traced to some fortuitous or accidental circumstance; and it belongs perhaps almost exclusively to the nineteenth century, to boast of some valuable discoveries, which, independent of chance, have resulted from scientific investigations and experiments directed to a specific object: Of these, the safety-lamp of Sir H. Davy, and his present chemico-electric guard to the copper of vessels, and the correcting plate of Professor Barlow, form memorable examples; and in all these cases, the value of the discovery is only equalled by the extreme simplicity of the application.

It is now at least 600 years since the compass began to be employed as a nautical instrument; and yet it is only within a short period, that an imperfection has been discovered in it, which detracts much from its real value for such a purpose, namely, that the needle does not continue to point in the same direction, with the ship's head at different points, the difference in some cases being so great as to lead to the most fearful errors and uncertainties. The general nature of this effect will be understood, by considering, that the upper parts of all iron bodies attract that end of the needle, which, when freely suspended, dips below the horizon, that is, the north end in the northern, and the south end in the southern, hemisphere. Now, as in vessels of all kinds, the iron which enters into their construction or equipment is nearly symmetrically disposed with respect to the axis, or longitudinal section, of the ship, when this axis is in the magnetic meridian of any place, the action of the iron is either coincident with, or directly opposed to, the magnetic action of the earth, and in either case the lateral direction of the needle is not disturbed: but when the ship's head is on any other point, and particularly towards the east or west, the two forces acting on the needle being nearly at right-angles to each other, the latter is drawn more or less out of its natural position, according to the direction of the resultant of these two forces; and the consequence of this is, that the course shown on shipboard by the compass, differs from the actual course of the vessel, by a quantity equal to the whole of this disturbing force, and which, in some cases, as we shall see, amounts to two or three points of the compass, that is, to 20°, 30° or 40°, but varying with every position of the ship's head, and with every change of terrestrial situation, from one pole of the earth to the other, and according to laws which till the present time seemed to bid defiance to every attempt made to unravel them. We have said, that it is only within a short period that this disturbing power has been observed; but it should be stated, that some obscure notices of such an effect are slightly alluded to by navigators of anterior date, as for example by Dampier, by Cook, and one or two French navigators. It does not however appear, that, in these instances, any thing more than the mere fact is stated, without any idea being thrown out respecting the cause, and much less of any remedy for the anomaly in question. The cause is, we believe, first distinctly noticed by Mr Downie, Master of His Majesty's ship Glory, in his report to the Admiralty, published in Walker's Treatise on Magnetism, in the year

1794. This experienced officer says, "I am convinced that the quantity and vicinity of iron in most ships, has an effect in attracting the needle; for it is found by experience, that it will not point in the same direction when placed in different parts of a ship: also it is rarely found that two ships steering the same course by their respective compasses, will go exactly parallel to each other; yet these compasses, when compared on board the same ship, will agree exactly."

A few years after this, the action of the iron of the vessel was more minutely noticed by Capt. Flinders, who was the first to trace its connexion with the dip of the needle, and to point out that the effect was different in quality on the contrary sides of the magnetic equator, and increasing in quantity as the dip in either hemisphere increased: and by him the subject was brought under the notice of the Admiralty Board, who ordered experiments to be made in different ships, in order to ascertain the general amount of the error thus produced. The inquiry was, however, again lost sight of, till Mr Bain published his valuable treatise on the "Variations of the Compass," in which the fatal consequences attending this source of error, are put in so clear a point of view, as to strike the most indifferent and uninformed readers; and whatever advances have since been made towards correction, the nautical profession will owe much of it to observations contained in this useful work. It happened, that, at this time, our arctic expeditions were in contemplation; and the local attraction of the vessels in those seas was one of the objects to which the attention of the officers was particularly directed. The results of the experiments made in these instances, are given by Captains Ross and Parry, in the accounts of their respective voyages; and the amount of the disturbing force was found to be such, as to call for some prompt and efficient remedy, the difference of the bearing of an object having been found by Captain Sabine to be at least 50°, merely from a change of position of the ship's head from east to west.

When the great amount of this error is thus pointed out, it will seem extraordinary that it should have remained so long unnoticed. It may, therefore, be proper to observe, in justice to the memory of the many excellent navigators, whose names and discoveries now only live in history, that this effect was much less formerly than it is at present, in proportion as the quantity of iron used in the construction and equipment of vessels was less than at this time. It is only within a few years that pig-iron has been employed for ballast, the weight of which,

in some vessels, exceeds three hundred tons; an immense surface of iron is also introduced, by the admirable invention of iron-tanks, to supply the place of the old water-casks. Moreover, the knees, sleepers, and, in some cases, even the riders are now of iron; hempen cables have been supplanted by those of iron; and some attempt has been recently made to employ gun-carriages of the same material. But of all innovations of this kind, the patent capstan by Captain Phillips, a highly valuable construction, has, perhaps, from its form and situation, the greatest effect on the compass: indeed, its action is so powerful, as will be seen as we proceed, that, without the means afforded by Mr Barlow's correcting plate, it must of necessity have been prohibited in all vessels of a smaller class than frigates. In the Griper, for example, the local attraction was 14° at east and west, making an extreme difference in the river Thames of 28°, which was reduced to about 16° by the removal of the capstan.

Having thus made our readers acquainted with the nature of the errors which nautical men had presented for philosophical investigation, let us follow Mr Barlow in the experiments he undertook, with a view to discovering some means of correction. It should be observed, that, at the time of which we are speaking, little or nothing was known of the mathematical laws of magnetic attraction. It had been ascertained, that, while a compass-needle was placed near the upper end of a bar of iron, the north end was drawn towards the bar, and that, near the bottom, the south end of the needle was attracted: and it consequently followed, that there must be some intermediate point, in which the effect of both ends was neutralized. It was also known, that a large mass of iron attracted more powerfully than a smaller mass; and that the effect was greater, as the distance between the iron and compass was less: and amongst other ratios it had been stated, that the power varied inversely as the cube of the distance; but still, no explicit and connected laws had been established: this, therefore, was the first object of the author. With this view, he procured a solid iron ball, thirteen inches in diameter and placing his compass above this, he found, as in the case of the bar, that the north end of the needle was attracted by the ball; that, when it was placed below it, the south end was attracted; and that, by causing his needle to descend in any vertical, it always passed through a point where the iron had no effect upon it. The question then occurred, Are all these points of no action in the same plane?

And, if so, is that plane parallel or oblique to the horizon? A series of experiments directed to this inquiry demonstrated, that the points were all in the same plane; and that this plane formed with the horizon an angle equal to the complement of the dip, descending from the north towards the south. That this ought to be the case when the needle had its natural dipping position, might easily have been foreseen; because, then, the iron would be symmetrically situated with respect to the two poles of the needle; but that it should still be the same with the horizontal needle, was a fact as novel to the author, as it was important in all his subsequent pursuits. Having traced this circle on his iron-ball, and assuming the direction of the dipping needle as a principal axis to the same, its extremities forming the poles, he was immediately in possession of an ideal magnetic sphere, by which to indicate the relative position of the iron and compass in all his future inquiries; and to this happy idea he is doubtless indebted for the remarkable success with which his experiments have been attended.

The nature and properties of this ideal sphere, and the facilities it affords in all magnetical computations, will be more readily comprehended by referring to Fig. 1. Pl. II., in which O is supposed to represent an iron-ball, and AAA a supposititious sphere circumscribing it, and within which its influence is active; SCNQ' being the magnetic meridian. The line NS in the plane SENW, denotes the natural direction of the dipping-needle in these latitudes where its inclination to the hori zon is about 70°. Now, conceiving QEQW to represent a circle or plane passing through the centre of the ball, and perpendicular to the axis NS, it will be the plane of no attraction or the magnetic equator; which, as we have seen, has this remarkable property, that if lines be drawn in it, (as for example, the lines OC, OC', OC", &c.), and a compass be placed any where in those lines, or, in short, in any point of the plane QEQ'W, it will be uninfluenced by the iron-ball, and preserve its natural magnetic direction. But, as soon as the compass is removed out of this plane, the needle is found to deviate from its original bearing; its south end being drawn towards the ball, when the needle is below the plane, and its north end when it is above; and in every case the deviation follows a determinate law, so that the amount being given in any one case, it may be computed for all others. Let us, for example, conceive any two other planes passing through the centre of the ball, and each perpendicular to the plane QEQ'W, of