the upper end gaining south polarity, and the lower end north.

8. Hot iron receives more magnetism of position than the same when cold.

9. A bar-magnet, if hammered when in a vertical position, or in the position of the magnetic axis, has its power increased, if the south pole be upward, and loses some of its magnetism if the north end be upward.

10. A bar of soft steel, without magnetic virtue, has its magnetism of position fixed in it, by hammering it when in a vertical position; and loses its magnetism by being struck when in the plane of the magnetic equator.

The application of magnetism to nautical purposes must now be noticed. The history of the compass is involved in much obscurity, as the Chinese appear to have possessed this important instrument long prior to its use in Europe. See COMPASS.

A common sewing-needle, rendered magnetical, and simply laid upon water, or fastened to a cork or straw, &c., and so laid upon water in a glass, or earthen or wooden cup; or else suspended by a very flexible thread, which must be fastened to its middle, forms a simple, but imperfect compass; and such, in all probability, was one of the first modes of constructing the compass, whence the magnetised steel wire, or bar, has ever since been called the magnetic needle. The defects of the last-mentioned construction are too evident to need any particular remarks. The needle laid upon water, is continually running to the sides of the cup, the water is liable to be spilt, or, if a thread be used, the stiffness of it will always influence the action of the needle.

A mariner's compass, or compass generally used on board of ships, is represented in fig. 8. The box, which contains the card or fly with the needle, is made of a circular form, either of wood, brass, or copper. It is suspended within a square wooden box by means of two concentric circles, called gimbals, so fixed by cross axes a, a, a, a, fig. 9, plate II., to the two boxes, that the inner one, or compass-box, shall retain an horizontal position in all motions of the ship, whilst the outer or square box is fixed with respect to the ship. The compass-box is covered with a pane of glass, in order that the motion of the card may not be disturbed by the wind. What is called the card is a circular piece of paper which is fastened upon the needle, and moves with it. Sometimes there is a slender rim of brass which is fastened to the extremities of the needle, and serves to keep the card stretched. The outer edge of this card is divided into 360 equal parts or degrees, and within the circle of these divisions it is again divided into thirty-two equal parts, or arcs, which are called the points of the compass, or rhumbs, each of which is often sub-divided into quarters. The initial letters N., N. E., &c., are annexed to those rhumbs, to denote the north, north-east, &c. The middle part of the card is generally painted with a sort of star, whose rays terminate in the above-mentioned divisions. To avoid confusion, those letters are not drawn in the figure.

The magnetic needles which are commonly used at sea are between four and six inches long;

but those which are used for observing the daily variation, are made a little longer, and their extremities point the variation upon an arch or circle, properly divided or affixed to the box.

The best shape of a magnetic needle is represented in figs. 1 and 2, plate II. MAGNETISM; the first shows the upper side, and the second shows a lateral view of the needle, which is of steel, having a hole in the middle, to which a conical piece of agate is adapted by means of a brass piece O, into which the agate cup (as it is called) is fastened. Then the apex of this hollow cap rests upon the point of a pin F, which is fixed in the centre of the box, and upon which the needle, being properly balanced, turns easily. For common purposes, those needles have a conical perforation made in the steel itself, or in a piece of brass which is fastened in the middle of the needle.

It is commonly imagined that the agitation of the compass is communicated by friction at the points or edges of suspension, and the compassmaker has accordingly exerted his ingenuity to diminish this friction, by contrivances similar to that of a conical cap balanced on a point, and itself affording another point to support the needle. But it may be readily proved by experiment, that the greatest disturbance of the needle is produced by the quantity of horizontal progressive motion, and not by the mere inclination or angular motion. A compass-needle, supported on a simple point, will suffer very little agitation from any angular motion, or moderate deviation from perpendicularity in the pin; but it will instantly begin to vibrate if moved horizontally. Thus the common experiment of tilting the compass-box in all positions, while its centre remains immoveable, is fallacious.

It appears, therefore, that the steadiness of a needle which vibrates slowly, is the consequence not only of the length of time it allows for alternate actions to operate, and destroy each other; but also of the difficulty with which it yields to such impressions. If the centres of suspension and of gravity in the needle were coincident, no angular motion would be produced by any action of the pin, excepting by the effects of friction; and the angular motion produced in other cases will be less, the shorter the distance between these two centres, or the lever by which it is propagated.

The simple suspension of the needle on a point has been applied to the compass-box, for which it is little suited, not only because of the wear upon so small a surface, but also because it admits the box to traverse horizontally; an effect which is inconvenient, and cannot be remedied by any means not calculated in some respect to increase the effects of agitation. The method most generally employed, and in fact the one best adapted to the purpose, consists in employing gimbals.

This well-known contrivance consists of a hoop supported upon two pins diametrically opposite to each other, and issuing from the external surface of the ring in such a direction that both lie in the same diameter line. When the hoop is suspended on these pins, it is at liberty to turn freely round the diameter of which they

constitute the prolongation. The notches or holes of support are disposed horizontally. The compass-box itself is placed in a similar ring, with two projecting pivots; and these pivots are inserted in holes made in the former ring at an equal distance from each of its pivots. If, therefore, we suppose the hole to be left at liberty, the compass-box may vibrate upon the diametral line of the outer ring, and also upon a line formed by its own pivots, at right angles to that diametral line. The consequence of this arrangement is, that the centre of gravity of the compass-box will dispose itself immediately beneath the intersection of both lines, on which it is at liberty to move-that is to say, if the weight of the box or its parts be properly disposed, the compass will assume a position in which its upper surface shall be horizontal.

The same principles which were applied to the single centre of the magnetic needle will also apply to the axis of the gimbals. If the centre of gravity of the compass-box be so placed with respect to either axis as that its vibrations shall be quick, every horizontal action will greatly disturb it, and it will not speedily settle. The most favorable position of the pivots or edges of support in the gimbals will be when they all lie in the same plane, and the centre of gravity of the compass-box is very little below that plane. The following valuable results, relative to the best mode of constructing the compassneedle, form part of an admirable paper, by captain Kater, inserted in the Transactions of the Royal Society:-1. That the best material for compass-needles is clock-spring; but care must be taken, in forming the needle, to expose it as seldom as possible to heat, otherwise its capability of receiving magnetism will be much diminished. 2. That the best form for a compass-needle is the pierced rhombus, in the proportion of about five inches in length to two inches in width, this form being susceptible of the greatest directive force. 3. That the best mode of tempering is, first to harden the needle at a red heat, and then to soften it from the middle to about an inch from each extremity, by exposing it to heat sufficient to cause the blue color, which arises, to disappear. 4. That in the same plate of steel, of the size of a few square inches only, portions are found varying considerably in their capability of receiving magnetism, though not apparently differing in any other respect. 5. That polishing the needle has no effect on its magnetism. 6. That the best mode of communicating magnetism to a needle appears to be by placing it in the magnetic meridian, joining the opposite poles of a pair of bar magnets (the magnets being in the same line), and laying the magnets so joined, flat upon the needle, with their poles upon its centre; then having elevated the distant extremities of the magnets, so that they may form an angle of about two or three degrees with the needle, they are to be drawn from the centre of the needle to the extremities, carefully preserving the same inclination; and, having joined the poles of the magnets at a distance from the needle, the operation is to be repeated ten or twelve times on each surface. 7. That in needles

from five to eight inches in length, their weights being equal, the directive forces are nearly as the lengths. 8. That the directive force does not depend upon extent of surface, but, in needles of nearly the same length and form, is as the mass. 9 That the deviation of a compassneedle, occasioned by the attraction of soft iron, depends, as Mr. Barlow has advanced, on extent of surface, and is wholly independent of the mass, except a certain thickness of the iron, amounting to about two-tenths of an inch, which is requisite for the complete development of its attractive energy.

The improved mariner's compass contrived by Mr. Pope, for which he has taken out a patent, is too important to be passed unnoticed. Its great merit consists in suspending the magnetic needle in such a manner as to combine the advantages of the dipping needle with those of the ordinary mariner's compass; the needle, from the peculiar mode in which it is suspended, being perfectly at liberty to dip in all latitudes, from the equator to the poles; while the card retains its horizontal position.

Fig. 3 represents the card, with the needle lying across it, in an orifice cut completely through the card from north to south. The card is perforated with a number of holes, to allow a free circulation of the air.

In fig. 4 the compass is seen in perspective, supported on an agate centre, and the needle dipping, while the card remains horizontal.

The advantages of this admirable invention are at once great and simple: in the first place, this compass increases its power of magnetic force where in others it diminishes, and that in every latitude from the equator to the poles; secondly, it retains its force even situated nearly over the polar point, where all others entirely lose it, and become useless; thirdly, it is more steady on its point, a circumstance of the first importance, when the agitation is increased by the motion of the vessel.

A few remarks by way of elucidation will prove the above; the first property it gains from its peculiar construction, by which the magnet is placed more in harmony with nature; and among many particulars, the principal point consists in the needle falling in a line with the attractive affluvia; this is effected by uniting the nature of a dipping-needle with that of a horizontal steering card; by virtue of which it has power to alter its own elevation, to every degree of latitude it may be placed in, and of course stands nearly in the line of attraction; this it performs without taking the card out of the horizontal position. The second it may be said to retain from the dip of the needle; for at the poles the dipping needle is found to increase its power, when that of the horizontal one is invariably lost. The third property it acquires by the manner of connecting the needle with the card; for when the motion is increased, and the compass exceedingly agitated, this needle quits the card, and attains a perpendicular motion, which serves most effectually to counteract that of the card; so that, between the motion of the card and the needle, the whole is brought to an equipoise; this in connexion with the card of the

compass being perforated, thereby admitting the air that is below the card, to pass freely above, without taking the card with it, renders it much steadier; and, being placed deeply in the box, it is not liable to strike against the glass like the common card.

It scarcely need be remarked that all compasses hitherto in use diminish their power every degree they are removed from the equinoctial line, which, the inventor of the present patent presumes, is occasioned solely by an erroneous system in the manufacture of the needle, as follows, viz. the mechanic is first careful to make it equally heavy at both ends, so as to swing parallel with the horizon on its point or centre; this done, the needle is further completed by touching it on the magnet for the purpose of gaining the magnetic poles; when it instantly requires a different position, falling from the horizontal to an oblique direction: to prevent this, and bring it parallel again, he is reduced to the alternative of loading the south end, either of the magnet or the card, which effectually performs the intention of the mechanic, but not of nature; for, thus encumbered, the needle is eventually so impeded, that, when it is brought near the poles, its efforts are fruitless, the card falling all around without any direction, from the single circumstance of being lifted beyond its natural inclination: and the magnet also is hereby drawn by main force above the magnetic line, and is obliged in its own defence to hang either on the left or right of it, in order to form a greater angle; for it is the distance from the poles only that enables the horizontal needle to act at all; and hence the variation may apparently be much increased hereby.

It may be suggested against this needle (from the fact that it is capable of being drawn nearly perpendicular when situated over the poles), that it will have no purchase or power over the card, and therefore cannot turn it round to its point: this would be correct but for the consideration of the poise of the needle, which always prevents a steering dipping-needle from falling to its due depression or dip, leaving a sufficient power for the purpose of turning the card; this is also provided for, by hanging the needle below its bearing.

The improvements in the new compass are evident, not only for the high north and south latitudes, but for the East and West India services, as being less subject to get out of repair and lose the power of the attractive force, and also more proof against local attraction, &c. This will appear more certain when it is recollected that all metallic substances (partly the cause of the variation) are contained nearer to the surface than the centre of the earth: they stand in the horizon; therefore, the common mariner's needle being poised horizontally, the metallic lodgments (in the horizon) have power to attract a horizontal needle, whereas this new dip ping steering needle points below that line, and forms nearly a true junction with the attractive influence, which is deep in the poles; hence this valuable instrument is not so subject to local attraction. Mr. Pope has adapted the principle

of the azimuth to the steering compass, which is so constructed as to be capable of steering by, and of taking an azimuth of the sun; thus making use of one compass alone, both to steer by and to take the azimuth.

This compass is furnished with a temporary verge (or glass) to shift on when steering, which leaves the face of the azimuth as clear to see the points and steer by as the common steering compass, and with the same facility performs both operations. He recommends it, therefore, to general use, in preference to his steering compass; but, when the latter is used, one only should be in the binnacle to steer by, instead of two as is customary at present; for there must be local attraction where two compasses are so close together.

In cases where the common compass becomes entirely useless, the able navigator is driven to the alternative of steering by the stars alone, which are not always to be seen, in consequence of thick weather, &c. For this reason, the addition of the dipping-needle to the mariner's compass is a matter of the first consequence to the instrument, and was never effected before the present time. From some experiments made by Mr. Pope, he is of opinion that the central part of the earth is one immense spherical mag net, extending from the centre to 40°, the remaining 50° being composed of strata surrounding that nucleus, and arranged from east to west. Now when the mariner's magnetic needle, which is a subject of the principal one (the earth), is brought up to latitudes above 60°, the magnetic north being deep in the earth, the dipping-needle in high latitudes is drawn almost perpendicular; and as it is now harnessed so as to be free of the card, and possesses both the advantages of the dip and of carrying the fly horizontally, its powers remain; whereas the old needle of the compass being riveted to the fly, and the fly being loaded at the south end to make it poise, it is thereby forced against its natural inclination, and brought up to a horizontal position. It then turns its back on its principal, the magnet of the earth; and herein, according to the inventor's opinion, consists the cause of the different effects of the old and new compasses.

Mr. Wales, who accompanied captain Cook in his second voyage, appears to have been the first person who observed the effects of the iron employed in ship building, upon the action of the magnetic needle. This subject was further investigated by captain Flinders, who, in his survey of the coast of New Holland, succeeded in applying a correction for the use of his own vessel. From an account of the survey, published some time after this navigator's return, it appeared that in captain Flinder's vessel, and indeed in every ship, a compass would differ very materially from itself, in being removed from the head to the stern. This was found to arise from the iron which surrounds the compass becoming magnetic, and its entire attractive force is thus concentrated into one powerful focus. Of this, the principal south pole is situ ated near the middle of the upper deck. This focus of attraction so influences the compassneedle, that it is subject to a considerable varia

tion from the true meridian, different from what is observed by a compass on shore; the north point of the compass being constantly drawn towards the focus in our hemisphere, and the south point in the opposite hemisphere.

With these facts in view, professor Barlow commenced a series of experiments to remedy the defective apparatus. Having procured a solid iron ball, thirteen inches in diameter, he found, on placing the compass above it, that the north end of the needle was attracted by the upper part of the ball; that, when placed below the ball, the south end was attracted; and that when the needle was raised or depressed, in any vertical around the ball, it always passed through a point in which both attractions became neutralised. The next step in his investigation was to ascertain whether these points of no attraction were all in the same plane; and, if so, to determine correctly its inclination to the horizon, since it had become obvious, from the care with which the trials had been made, that it was not parallel to it. This question was soon decided: by a series of experiments it was demonstrated that the points were all in the same plane, and that the inclination of the plane itself to the horizon was about 20°, declining directly from the magnetic north point to the south, approaching very nearly to the complement of the dip of the needle. This circle Mr. Barlow now traced on his iron ball, assuming as its principal axis the direction of the dipping needle; and, imagining circles of latitude and longitude to be described around the whole ball, he had thus an ideal magnetic sphere, which would readily indicate the relative position of the iron and the compass in his subsequent experiments. Plate II., fig. 5, will enable the reader to form a correct idea of the nature and properties of this magnetic sphere. O is supposed to represent an iron ball, and AAA a sphere circumscribing it, and within which its influence is active; S, N, being in the magnetic meridian. The line NS, in the plane S, E., N, W, denotes the natural direction of the dipping needle, in those latitudes where its inclination to the horizon is about 70° 30′. Now, conceiving Q, E, Q', W, to represent a plane, passing through the centre of the ball, and being perpendicular to the axis NS, it will be the plane of no attraction, which has this remarkable property, that if lines be drawn in it (as for example,the lines O C, O,C', O C, &c.) and a compass be placed any where in those lines, or indeed in any point of the plane Q, E, Q',W, it will be uninfluenced by the iron ball, and will preserve its natural magnetic direction.

As soon, however, as the compass is removed out of this plane, the needle is found to deviate from its original bearing,-its south end being attracted 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 ascertained for all others.

Suppose, for example, any two other planes, passing through the centre of the ball, each being perpendicular to Q, E, Q', W, of which let M,

O, S, L, represent quadrants; then supposing a compass placed in each of these planes, at equal distances from the centre, at L, we shall have ML, for the latitude, and E M, E M', for the longitude of position of those points; then will the following proportion express the law in question :-The tangent of the deviation of the compass at L is, to the tangent of the deviation of the compass at L', as the rectangle of the sine of 2 L M + cosine E M to the rectangle of the sine of 2 L'M' + cosine of E M', E, being the east point of the horizon.

Pursuing this enquiry, Mr. Barlow next ascertained, by a series of experiments, the law of attraction at different distances of the compass from the iron ball; and which he thus states:That, while the position as to latitude and longitude is the same, the tangents of the angles of deviation are reciprocally proportional to the cubes of the distances.

Still, however, there remained an important question to be solved, viz. when the position and distance are the same, what is the law of deviation as it respects the mass of the attracting body? And here a most unexpected result was obtained. From the first experiments, which were made with a solid iron ball of ten inches in diameter, it appeared, as Mr. Barlow seems to have anticipated, that the tangents of the deviations are proportioned to the cubes of the diameters, all other things being the same; but happening at this time to make trial of an iron wheel of the same diameter as the ball, but only three-fourths of its weight, he was not a little surprised to find the results in both cases the same. 'In fact,' he remarks, it appeared that the power of attraction resided on the surface, and was independent of the mass.'

Before it was discovered that the power of an attracting body resides in its surface, Mr. Barlow had anticipated a great impediment in the way of final success, arising from the mass of iron which he considered as necessary to produce the desired effect. It now, however, readily occurred to him, that, as surface is the principal thing to be attended to, a light, globular shell of iron, or a simple circular plate of the same metal, would be amply sufficient for the purpose. A plate of this description was accordingly tried; it was fifteen inches in diameter, and weighed only 4lbs. 13oz. With this plate Mr. Barlow repeated the series of experiments that had been made with the ball, and had the satisfaction of finding that its power was far greater than would be requisite for doubling the effect of the guns of any vessel in the navy, although applied to the exterior of the binnacle, and fifteen inches distant from the pivot of the needle. The precise situation for this correcting plate is, of course, a matter of essential importance, and cannot be fixed on until the local attraction of the vessel has been ascertained. This is effected in the following manner:-The ship being so moored as to admit of her head being directed to each point of the compass successively, and there steadied whilst the bearing of as remote an object as can be found is taken, it will then be found that the bearings thus taken vary from each other, according to the attractive power of

the vessel, sometimes as much as 28°, which difference is caused by the iron of the ship drawing the needle from its proper direction to the eastward with the ship's head east, and to the westward with the head to the west. It will also be found that two of the bearings, taken at opposite points of the compass, nearly agree with each other, and the mean of these is to be taken as the true magnetic bearing of the object. By these points, also, will be indicated the line of no attraction in the vessel, which in general is found to be nearly fore and aft, and in this line the plate is ultimately to be placed. This being accomplished, a pedestal or compass-stand is then taken ashore, and by trying different situations for it, and turning it about till the same deviations are produced by the plate at each point, as had been observed in the vessel. But this process, which is very troublesome, is now rendered unnecessary, the end being much more readily accomplished by means of a printed table, which is given with the correcting plate; and which comprises a series of attractions obtained by the plate, including all possible limits for every class of vessels. Corresponding to each degree of local attraction are given two numbers, the one indicating the distance of the centre of the plate below the pivot of the needle, and the other its distance from the centre line of the pedestal, and at this depth and distance the plate must be fixed, either fore or aft the compass; if the former, the effect of the vessel will be exactly doubled, and if the latter it will be neutralised. When the plate is applied in front, which is reckoned preferable in southern voyages, it is not a fixture, but is applied at pleasure; but when placed behind, as recommended in northern voyages, where the disturbance from local attraction is very considerable, it is fixed in its place during the voyage, and the needle is thus left free to obey the influence of the magnetic power of the earth only.

It now remains to give some account of the plate itself. The plate employed by Mr. Barlow in his experiments, as well as those which he sent out by captains Parry and Sabine, consisted of two circular pieces of sheet iron, weighing about 3lbs. per square foot, screwed together in such a manner as to combine any strong irregular power of one plate, with a corresponding weak part on the other, by which means a more uniform attraction is produced; he is, however, of opinion that a single plate of iron, weighing about 6lbs. per square foot, would answer the purpose equally well. In diameter the plate may vary from twelve to sixteen inches, according to the power of the vessel. When the plate is made double, there is a circular board of the same size inserted between the iron discs, for the purpose of increasing the thickness, without adding much to the weight. The two plates thus separated are found to be more powerful than either a single plate, or two plates in immediate contact. The plates are perforated in the centre, and through this opening is passed a brass socket with a broad head, and having an exterior screw and nut, by which the two iron plates, and the interposed plate of wood, are compressed together. Fig. 6 represents the

brass pin, socket, plate, pedestal, and compass, combined, as in action on ship-board.

It appears from a curious paper by Mr. Fisher, on the errors in longitude as determined by chronometers at sea, that a sudden alteration takes place in their rate when taken on shipboard, an effect which has been generally ascribed to the motion of the vessel. He ascribes the acceleration which takes place to the 'magnetic action exerted by the iron in the ship, on the inner rim of the balance, which is made of steel;' and in proof of this, he found that analogous effects took place in chronometers when under the influence of magnets placed in different positions with respect to their balances. Upon the whole,' says Mr. Fisher, 'it appears that chronometers will be generally accelerated (particularly if their balances have received polarity by the too near approach of any thing magnetical) on ship-board. It appears probable, likewise, that the force of the balance-springs is affected in the same way, since it is well known that chronometers having gold balance-springs, although more difficult to adjust, yet keep better rates at sea than others.'

The use of the chronometrical compass stand, contrived by captain Scoresby, is to preserve the rate of a chronometer, although magnetic properties may have been imparted to it. The instrument represented in fig. 7 consists of a slender cross of brass, supported on a long point of brass, or steel, in a compass-bowl, from which is suspended a rhomboidal compass-needle. On the centre of the brass cross there is a light case of card paper, fitted to the pocket chronometer, to be carried by it. The case for the chronometer slips upon two pins, riveted to a moveable plate upon the cross, which, being made to slide in different directions, may be so placed as to adjust the chronometer fairly over the centre of the needle, and is then fastened by screws. In this state, the magnetic needle below it causes the cross and chronometer to traverse with great celerity. It therefore has the property of keeping the chronometer invariably in the same position, and, being suspended on gimbals, of preserving it from the bad effects of the motion of the ship at sea. The magnetic needle was hung five or six inches below the chronometer, so that its influence on the instrument was not greater than that of the earth; and, being in an opposite direction, has a tendency to neutralise rather than add to this disturbing cause.

The azimuth compass is nothing more than the mariner's compass already examined, to which two sights are adapted, throughout which the sun is to be seen, in order to find its azimuth, and thence to ascertain the declination of the magnetic needle at the place of observation. The particulars in which differs from the usual compass are, the sights FG hg. 8; in one of which, G, there is an oblong aperture with a perpendicular thread or wire stretched through its middle; and in the other sight, F, there is a narrow perpendicular slit. The thread or wire, HI, is stretched from one edge of the box to the opposite. The ring A B of the gimbals rests with its pivots on the semi-circle CD, the foot, F, of which turns in a socket, so that whilst the box,