or Mariner's Steering Compass, is an instrument used at sea by pilots to direct and ascertain the course of their ships. It consists of a circular brass box, which contains a paper card with the 32 points of the compass, fixed on a magnetic needle that always turns to the north, excepting a small declination variable at different places. See Variation.

The needle with the card turns on an upright pin fixed in the centre of the box. In the centre of the needle is fixed a brass conical socket or cap, whereby the card hanging on the pin turns freely round the centre.

The top of the box is covered with a glass, that the card's motion may not be disturbed by the wind. The whole is inclosed in another box of wood, where it is suspended by brass hoops or gimbals, to preserve the card horizontal. The compass-box is to be so placed in the ship, that the middle section of the box, parallel to its sides, may be parallel to the middle section of the ship along its keel.

The compass being of the utmost consequence to navigation, it is reasonable to expect that the greatest attention should be used in its construction, and every attempt to improve it carefully examined, and, if proper, adopted. But so careless are the generality of commanders of this most useful instrument, that almost all the compasses used on board merchant-ships have their needles formed of two pieces of steel-wire, each of which is bent in the middle, so as to form an obtuse angle; and their ends, being applied together, make an acute one; so that the whole represents the form of a lozenge; in the centre of which, and of the card, is placed the brass-cap. Now, if we examine a number of these cards, we shall rarely, if ever, find them all in the same direction, but they will all vary more or less, not only with regard to the true direction, but from one another.

These irregularities are owing to the structure of the needle; for the wires of which it is composed are only hardened at the ends; now, if these ends are not equally hard, or if one end be hardened up higher than the other, when they come to be put together, in fixing them to the card, that end which is hardest will destroy much of the virtue of the other; by which means the hardest end will have the most power in directing the card, and consequently make it vary toward its own direction: and, as the wires are disposed in the form of a lozenge, these cards can have but little force, so that they will often, when drawn aside, stand at the distance of several degrees on either side the point from whence they are drawn: for all magnetic bodies receive an additional strength by being placed in the direction of the earth's magnetism, and act proportionably less vigorously when turned out of it; therefore, when these kind of needles are drawn aside from their true point, two of the parallel sides of the lozenge will conspire, more directly than before, with the earth's magnetism; and the other two will be less in that direction: by which means the two sides will very much impede its return; and the two latter will have that impediment to overcome, as well as the friction, by their own force alone.

To remove these inconveniences, some needles are made of one piece of steel of a spring temper, and broad towards the ends, but tapering towards the middle, where a hole is made to receive the cap. At the ends they terminate in an angle, greater or less according to the skill or fancy of the workman. These needles, though infinitely preferable to the other, are, however, far from being perfect; for every needle of this form hath six poles instead of two, one at each end, two where it becomes tapering, and two at the hole in the middle; this is owing to their shape; for the middle part being very slender, it has not substance enough to conduct the magnetic stream quite through, from one end to the other: all these poles appear very distinctly, when examined with a glass that is sprinkled over with magnetic sand. This circumstance, however, does not hinder the needle from pointing true; but as it has less force to move the card than when the magnetic stream moves in large curves from one end to the other, it is certainly an imperfection.

These inconveniences induced the ingenious Dr Knight to contrive a new sea-compass, which came into use on board all the ships of war. The needle in this instrument is quite straight, and square at the ends; and consequently has only two poles, though about the hole in the middle the curves are a little confused. Needles of this construction, after vibrating a long time, will always point exactly in the same direction; and if drawn ever so little on one side, will return to it again, without any sensible difference. We may therefore conclude, that a regular parallelopiped is the best form for a needle, as well as the simplest, the holes for the caps being as small as possible.

And as the weight should be removed to the greatest distance from the centre of motion, a circle of brass, of the same diameter of the card, may be added, which will serve also to support the card, which may then be made of thin paper, without any thing to stiffen it. This ring being fixed below the card, and the needle above it, the centre of gravity is placed low enough to admit of the cap being put under the needle, whereby the hole in the needle becomes unnecessary.

The above observations will be easily understood from viewing the several parts of the instrument as represented on Plate CXLIV. where fig. 6. is the card, with the needle K.L., and its cap M., fixed upon it, being one third of the diameter of the real card. Fig. 8. is a perspective view of the backside of the card, where A.B. represents the turning down of the brass edge, C the under part of the cap, D and E two sliding weights to balance the card, and F, G, two screws that fix the brass edge, &c., to the needle. Fig. 7. is the pedestal that supports the card, containing a screwing needle, fixed in two small grooves to receive it, by means The invention of the compass is usually ascribed to Flavio da Meli, or Flavio Gioia, a Neapolitan, about the year 1302; and hence it is, that the territory of Principato, which makes part of the kingdom of Naples, where he was born, has a compass as its arms.

Others say that Marcus Paulus, a Venetian, making a journey to China, brought back the invention with him in 1260. What confirms this conjecture is, that at first they used the compass in the same manner as the Chinese still do; i.e., they let it float on a little piece of cork, instead of suspending it on a pivot. It is added, that their emperor Chingus, a celebrated astrologer, had a knowledge of it 1120 years before Christ. The Chinese only divide their compass into 24 points. Fanchette relates some verses of Guoyot de Provence, who lived in France about the year 1200, which seem to make mention of the compass under the name of marinette, or mariner's stone; which show it to have been used in France near 100 years before either the Melite or Venetian. The French even lay claim to the invention, from the fleur de lys wherewith all nations shall distinguish the north point of the card.

With as much reason Dr Wallis ascribes it to the English, from its name compass, by which name most nations call it, and which he observes is used in many parts of England to signify a circle.

Though the mariner's compass has been long in use, the best construction of it was attended with many inconveniences, till the late improvement which it has received from the invention and experiments of Dr Gowin Knight, and the farther emendation of Mr Smeaton, as has been described under the article AZIMUTH (Vol. II.)

The compass has sometimes been observed to be disturbed by the electricity of its glass cover; and this from so slight an application of the finger as was barely necessary to wipe off a little dust. The same glass, rubbed a little more with the finger, a bit of mullin, or paper, would attract either end of the needle, so as to hold it to the glass for several minutes, far out of the due direction, according to that part of the glass which was most excited. And when the needle, after adhering to the glass, has dropped loose, and made vibrations, those would not be affected as usual by that point where the needle should rest, but would either be made all on one side, or be very unequally divided, by means of some remains of electrical virtue in that part of the glass which had attracted the needle, until at length, after 15 minutes or more, all the electricity being discharged, the magnetic power took place. The remedy for this inconvenience is to moisten the surface of the glass; a wet finger will do it immediately and effectually. The mariner's compass with a chart is much less dangerously moved than the common compass with a bare needle: and the deeper, or farther distant, the needle hangs below the glass, the less disturbance it is likely to receive.

Notwithstanding the various contrivances that have been made to prevent the card from being much affected by the motions of the ship, they have always been found too delicate to encounter the shocks of a tempestuous sea. Improved sea-compasses have lately been constructed by Mr McCulloch of London (and for which he has obtained a patent), that are reported to be the best of any yet used. The particulars are as follow:

Fig. 1. is a section of the steering compass. Aaaaa, The common wooden-box, with its lid. bb, The brass compass-box. cc, The glass cover to ditto. dd, The hollow conical bottom. ee, The prop upon which the compass is supported instead of gimbals; the spherical top of which is finely polished, and the apex of the hollow cone is fitted in a peculiar manner to receive it. ff, A quantity of lead run round the bottom and cone of the compass box, to balance and keep it steadily horizontal. gg, The card and the magnetic needle, bent in such a manner that the point of the conical pivot on which it moves and is supported, may be brought very near to the centre of gravity, as well as to the centre of motion. hh, Two guards, which by means of two pins, affixed to the compass box, prevents it from turning round and deceiving the steerman.

Fig. 2. a perspective view of the steering compass, with the lid off and the front laid open. kk, The guards. b, The compass-box. ee, The prop, &c. as in fig. 1.

Fig. 3. a view of the azimuth compass. b, The compass-box. h, One of the guards, ee, The prop, as in fig. 1. and 2. with this difference, that in the azimuth compass, instead of being screwed to the bottom of the wood-box, stands in a brass socket, and may be turned round at pleasure. 1. A brass bar, upon which the sight vanes are fixed. 2. A dark glass, which moves up or down on 3. the sight vane. 4. A magnifying glass, which is also moveable on the other vane. 5. The nonius or vernier. 6. A slide for moving the vernier so as to stop the card in taking the azimuth. 7. A double convex glass, by which the divisions on the vernier may be read with accuracy.

Fig. 4. is a section representing another application of the magnetic needle and card, constructed by Mr McCulloch. Aaaaa, The common wood-box. bb, The brass compass-box. cc, The brass support for the circle and pendulum. d, The pendulum. ee, The agate. ff, The magnetic needle and card. gg, The brass circle. hh, The glass cover and brass ring. ii, The lead weight. N.B. All the centres of motion are in the same plane.

"In one particular this patent compass is considered as an improvement on the common compasses, in as far as the needle is both longer and broader; hence its magnetism must be stronger, and of course the line of its magnetic direction correspondent with the card. In another particular, in order to prevent the motions of the vessel from affecting the needle, which is the most desirable object, the patent compass-box, instead of swinging in gimbals at right angles to each other, is supported in its very centre upon a prop; and whatever motion the other parts of the box may have, this centre being in the vertex of the hollow cone, may be considered as relatively at rest; and therefore gives little or no disturbance to the needle. Again, the pivot or centre upon which the needle turns, is so contrived as to stand always perpendicular over the centre of the compass-box, or apex of the hollow cone, as upon a fixed point; and is therefore still less affected by..." by the motions of the vessel. Thus the centres of motion, gravity, and of magnetism, are brought almost all to the same point; the advantages of which will be readily perceived by any person acquainted with mechanical principles. Experience therefore will ascertain the utility of this improvement." Mr. Culloch's Account.

Of Dr Knight's Azimuth Compass, as improved by Mr Smeaton, a description was given under the article Azimuth, and a figure in Plate LXXVII. The use of the azimuth compass is for finding the sun's magnetic azimuth, or amplitude; and thence the variation of the compass. If the observation be for an amplitude at sun-rising, or for an azimuth before noon, apply the centre of the index on the west point of the card, within the box; so that the four lines on the edge of the card, and those on the inside of the box, may meet. If the observation be for the sun's amplitude setting, or an azimuth in the afternoon, turn the centre of the index right against the east point of the card, and make the lines within the box concur with those on the card: the instrument thus fitted for observation, turn the index towards the sun, till the shadow of the thread fall directly on the slit of the sight, and on the line that is along the middle of the index: then will the inner edge of the index cut the degree and minute of the sun's magnetic azimuth from the north or south. But note, that if, when the compass is thus placed, the azimuth is less than 45° from the south, and the index turned towards the sun, it will pass off the divisions of the limb: the instrument therefore in this case must be turned just a quarter of the compass, i.e., the centre of the index must be placed on the north or south point of the card, according as the sun is from you; and then the edge will cut the degree of the magnetic azimuth, or the sun's azimuth from the north, as before.

The sun's magnetic amplitude thus found, the variation of the needle is thus determined. Being out at sea the 15th of May 1717, in 45° north latitude, the tables give me the sun's latitude 19° north, and his east amplitude 27° 25' north: by the azimuth compass, I find the sun's magnetic amplitude at his rising and setting; and find he rises, v.gr., between the 62d and 63d degree, reckoning from the north towards the east point of the compass, i.e., between the 27th and 28th degrees, reckoning from the east. The magnetic amplitude, therefore, being here equal to the true one, the needle has no variation; but if the sun at his rising should have appeared between the 52d and 53d degree from the north towards the east; his magnetic amplitude would then have been between 37 and 38 degrees, i.e., about 10 degrees greater than the true amplitude: therefore, the needle would vary about 10 degrees north-easterly. If the magnetic east amplitude found by the instrument should be less than the true amplitude, their difference would show the variation of the needle easterly. If the true east amplitude be southward, as also the magnetic amplitude, and this last be the greater; the variation of the needle will be north-west; and vice versa.

What has been said of north-east amplitudes holds also of south-west; and what of south-east amplitudes holds of north-west amplitudes. Lastly, if amplitudes be found of different denominations, v.gr., if the true amplitude be six degrees north, and the magnetic amplitude five degrees south; the variation, which in this case is north-west, will be equal to the sum of the magnetic and true amplitudes: understand the same for west amplitudes.

The variation may likewise be found from the azimuth: but in that case, the sun's declination, latitude of the place, and his altitude, must be given, that his true azimuth may be found.

This instrument is also useful in settling the ship's wake, in order to find the lee-way; and also to find the bearings of head-lands, and other objects.

Compass is also an instrument of considerable use in surveying land, dialing, &c.

Its structure, in the main, is the same with that of the mariner's compass; consisting, like that, of a box and needle: the principal difference consists in this, that instead of the needle's being fitted into the card, and playing with it on a pivot, it here plays alone; the card being drawn on the bottom of the box, and a circle divided in 360 degrees on the limb. See fig. 5. This instrument is of obvious use to travellers, to direct them in their road; and to miners, to show them what way to dig, with other considerable uses.

1. To take the declination of a wall by the Compass. Apply that side of the compass whereon the north is marked along the side of the wall; the number of degrees over which the north end of the needle fixes will be the declination of the wall, and on that side; v.gr., if the north point of the needle tends towards the north, that wall may be shone on by the sun at noon; if it fix over fifty degrees, counting from the north towards the east, the declination is so many degrees from north towards east.

But since the needle itself declines from the north towards the west, with us, 13°; it must be noted, that to retrieve the irregularity, 13° are always to be added to the degrees shown by the needle, when the declination of the wall is towards the east; on the contrary, when the declination is towards the west, the declination of the needle is to be subtracted.

2. To take an angle with the Compass. Suppose the angle required be DAE, fig. 4; apply that side of the compass whereon the north is marked to one of the lines AD; when the needle rests, observe the degrees at which its north point stands, which suppose 80°: so many degrees does the line decline from the meridian. In the same manner take the declination of the line AE, which suppose 215°; subtract 80° from 215°, the remainder is 135°; which subtracted from 180°, there will remain 45°; the quantity of the angle required. But if the difference between the declination of the two lines exceed 180°; in that case, 180° must be subtracted from that difference: the remainder then is the angle required.

In measuring angles by the compass, there needs not any regard be had to the variation; that being supposed the same in all the lines of the angles.

3. To take a plot of a field by the Compass. Suppose the field A, B, C, D, E, fig. 10. For the greater accuracy let there be two sights fitted to the meridian line of the compass, place it horizontal, and through the sights look along the side AB, or a line parallel to it; applying the eye to the sight at the south point of the compass. Draw a rough sketch of the field by Compass, the eye, and on the corresponding line enter down the degree to which the needle points, which suppose 90; measure the length of the side, and enter that too, which suppose 10 chains.

In this manner proceed with all the rest of the sides and angles of the field; the sides, which suppose 70, 65, 70, 44, 50 fathom; and the angles, which suppose 30, 100, 130, 240, 300 degrees. To protract the field, set down the several angles observed, one after another, and subtract the lesser from the next greater: thus will you have the quantity of the several angles, and the length of the lines that include them. For the rest, see Geometry.

Note, All the angles of the figure taken together, must make twice as many right angles; abating two if no mistake has been committed.

Azimuth Compass. See Azimuth.

Compass-Dials are small horizontal dials, fitted in brass or silver boxes, for the pocket, to show the hour of the day, by the direction of a needle that indicates how to place them right, by turning the dial about till the cock or style stand directly over the needle; but these can never be very exact, because of the variation of the needle itself. See Compass, and Dialing.