tion for his successful experiments, we must be permitted to think that his analytical calculations are of little value, and quite as likely to mislead as to direct the course of our inquiries.

"These experiments suggested at once a remedy for the errors due to the local attraction of ships; for the action of any mass of iron may be referred to two points indefinitely near each other in the general centre of attraction of the masses of iron on board. If, therefore, in the line joining this centre and the needle, we place on the opposite side a mass of iron, whose action on the needle shall be just equal to that of the disturbing force of the vessel, these forces being opposite will destroy each other, and leave the needle at liberty to obey the action of the earth's magnetism. Experiment soon showed that a small plate of iron placed within a few inches of the compass was sufficient to produce this effect. This was Professor Barlow's first suggestion to the Admiralty.

"The first experiments with the correcting plate were made on board his majesty's ship Leven, which sailed under the command of Captain Bartholomew, in 1820, to the western coast of Africa, but returned the following year under the command of Captain Baldey in consequence of the death of the former officer. A very extensive series of observations led to the most satisfactory results.

"It was obvious, indeed, without any such practical determination that this must have been the case; but still from that distrust with which practical men always regard the discoveries of abstract investigation, this remedy could only be classed with the dreams of theorists till confirmed by actual experiment. Two cases of a decided character had occurred very recently, which seemed to furnish an experimentum crucis, and on these it was resolved to try the operation of the correcting plate.

"Captain Flinders had observed that with an equal north and south dip, he found an equal quantity of deviation, but in a contrary direction. To see whether the plate would meet these circumstances was the point left for the decision of Captain Basil Hall, in his voyage in the Conway round Cape Horn to the western coast of America. Observations were accordingly carried on from England below Cape Horn to the latitude of 61° south, and throughout this great arc of terrestrial latitude the results are the most satisfactory that can be desired.

"The next point to be settled was this. It had been ascertained by the observations of Captains Ross and Parry, that the effect produced by the iron of the ship had increased with immense rapidity in approaching towards the pole. Would the power of the plate increase with rapidity? It seems to us that not a shadow of doubt

could have been rationally entertained; but, to make assurance doubly sure,' Lieutenant Foster, who had already received the thanks of the Board of Longitude, for his experiments on this and other scientific subjects in the Conway, was now appointed to the Griper, which was about to leave England for Spitzbergen under the command of Captain Clavering. His experiments were the more interesting, that they were made in very high latitudes where hitherto the compass had been generally stowed away as useless, both on this account as well as from the circumstance of the ship's local attraction being much greater than usual. By observations made while the vessel was lying at the Nore, the bearing of an object was found to differ 28° with the ship's head at east and west. That is, the local attraction was 14° at each of these points, and proportionally great in all intermediate positions, an amount of deviation truly astonishing, and which Captain Clavering ascribed to the influence of the spindle of the patent capstan, a suggestion which was verified by experiment on the return of the vessel, as we have already stated. To counteract this strong power it was necessary to bring the iron plate which was 14 inches in diameter, to a distance from the middle of the pedestal of 73 inches, and the centre of it 71 inches below the pivot of the needle, in which situation abaft the compass, it balanced the local attraction of the ship and left it free to obey the natural directive power of the earth; this was proved by taking the variations of the compass with and without the plate from England to the North Cape, when the close agreement of the former and the great discrepancy in the latter were so marked, that the vessel was navigated during the remainder of the voyage altogether by the corrected compass.

"The Griper was swung at three different ports during the voyage; at Drontheim, Hammerfest, and Spitzbergen, and the local attrac tion ascertained at every station, first with and then without the plate. With the plate the deviations were reduced to quantities very little exceeding what might be attributed to errors of observation; without the plate they were found to be at the east and west, or maximum points, as follows;

"The nature however of these irregularities, and the importance of Professor Barlow's plate will be more distinctly seen from the following table of variations with and without the plate, taken during the voyage.

"The uniformity of the change in the variation when the correcting plate was employed is obvious at a single glance; whereas the rapid and large irregularities which are shewn, when the plate was not used, placed in the strongest light its great importance. Thus we see on the 18th May, by simply warping the ship round from N. to NE. the variation experienced a change of 15°; on the 20th, by a change from N. to E. N., the variation was reduced from 24° 52′ to 2o 14'; and lastly, on the 28th, the change of direction in the ship's head from N. E. to W. produced an increase of nearly 30° in the variation. These are not solitary instances. The log-book presents a continued succession of them. Under such circumstances, it is obvious that the compass becomes a mere piece of useless furniture.”

"Every reader," says Professor Barlow, "whether a nautical man or not, must be aware of the great amount of error and fatal consequences which might arise in a few hours to a vessel in the channel, in a dark and blowing night, having for its only guide a compass subject to an error of 14o in opposite directions at east and west, the very courses on which she would be endeavouring to steer; and who can say how many of the mysterious wrecks which have taken place in the channel are to be attributed to this source of error, of which the most recent, that of the Thames, Indiaman, is a serious example. This vessel, besides the usual materials, guns, &c., had a cargo of more than 400 tons of iron and steel; and it may be easily imagined, that such a cargo would produce an effect on the compass at least equal to that of the Griper and Barracouta; and this alone would be quite sufficient to account for the otherwise inexplicable circumstance, that after having Beachy Head in sight at six o'clock in the evening, the vessel should have been wrecked upon the same spot at one or two in the morning without the least apprehension of being at all near shore."-See Barlow's Magnetic Attractions, 2d ed.

[In the above note the compiler has availed himself of an abstract and some remarks contained in the Westminster Review for April 1825.]

VII.

Theory of Magnetism, by M. Poisson. See Annales de Chimie, pour Févier, 1824.

"THE first step in this inquiry was obviously to reduce to three rectangular co-ordinates the results of all the attractions and repulsions exerted by the magnetic elements of a magnetized body of any imaginable form upon a given point, situated either within or without the body. By adding to these results, as belonging to any point within the system, those of the external magnetic forces that act upon the body, we have the whole forces that tend to separate the two fluids which are united at the point in question. And if the matter of the body opposes no resistance to the displacement of the two fluids; or, in other words, if there be no coercive force, it will be necessary, in order that there may be an equilibrium, that all the attractions and repulsions should destroy each other. The sum of the forces, therefore, in the directions of these three co-ordinates are severally made equal to zero. These equations are at first, as might be expected, somewhat complicated; but by means of certain transformations, the triple integrals, in terms of which they are expressed, are reduced to double integrals, and the equations very considerably simplified. From these equations M. Poisson has been able to deduce the following general principles, remarkable for their singular simplicity, novelty, and beauty.

I. That, notwithstanding the boreal and austral fluids are distributed throughout the mass of a body, magnetized by induction, the attraction and repulsion, which it exerts externally, are the same as if it were merely covered by a very thin stratum, formed of the two fluids in equal quantities, and such that their total action upon all the points within them should be equal to nothing. This theorem extends to all bodies whatever.

II. When the general formulas of this memoir are applied to a hollow sphere of uniform thickness, the following remarkable result is obtained;— A magnetic needle, placed within a hollow sphere of soft iron, and so small as not to exert any sensible influence on the sphere, will not be subject to any magnetic action, and will consequently not exhibit any polarity, from the effect of the earth's magnetism, or from that of any other magnet placed without a hollow sphere.” We need not stop to point out the striking analogy between this result and the case of a material particle placed within a hollow shell of matter attracting according to the general law of gravitation.

III. If the general formulas be applied to the particular case of a sphere magnetized by the action of the earth, they admit of being integrated in finite terms, and of being completely resolved. We are, therefore, enabled to determine every thing relative either to the direction of the line of polarity, or the intensity of the magnetism in the solid part of the sphere, or its action on any point without, given in position. In this case, although the magnetism is not confined to the exterior surface of the hollow sphere, and although its intensity may be determined for any point of the hollow shell, yet the magnitude of the three component forces, produced by it, is wholly independent of the thickness of the metal,-it is determined simply by the radius of the external surface and the co-ordinates of the point on which the forces act. When the distance of this point from the centre of the sphere is very great compared with the radius, each of the three forces is very nearly proportional to the cube of the radius directly, and the cube of the distance inversely. These forces may be reduced to two, a force to, or from, the centre of the sphere, and a force in the direction of the dipping needle. The former of these vanishes when the point is situated in the plane passing through the centre of the sphere and perpendicular to the direction of the latter force. Hence, if a small magnetic needle be placed in this plane, the direction which it would assume by virtue of the action of the earth will not be altered by the attraction of the sphere. We must not, however, infer that the attraction vanishes in this plane; for the second force does not vanish at the same time with the first; it will be subtracted from the first, and its effect will be to retard more and more the oscillations of the needle, as it is brought nearer the surface of the sphere. At the surface itself, and in any plane intersecting it, this force is equal and contrary to the action of the earth; so that in this situation the small needle will only be urged in the direction of the radius; and, provided it were so small that its action on the sphere would be inconsiderable, in the plane perpendicular to the dipping needle, and very near the surface of the sphere, the needle would be exempt from all magnetic action, and would have no determinate position.

"M. Poisson has announced his intention to investigate in a second memoir the laws which regulate the distribution of magnetism in needles of steel magnetized to saturation, and in needles of iron magnetized by induction, by means of the general formulas which have been demonstrated; and from these distributions to deduce the phenomena of their mutual attractions and repulsion."