A nautical instrument, showing the direction in which a vessel is sailing, or the part of the horizon to which her head is directed. It consists of a flat bar of steel, which being repeatedly rubbed with a magnet or loadstone acquires the property of constantly taking a direction nearly corresponding with the meridian, when freely suspended from a thread, or upon a delicate pivot. Although as important consequences have flowed from the invention of the compass as from any other invention whatever, the name of the author of it, and even the age and country in which it had its origin, are points involved in the greatest obscurity. In Europe we are indebted for its use to B. Givaia, of Naples, who, towards the close of the twelfth, or about the commencement of the thirteenth century, observed that iron became polarized by contact with the loadstone; but most writers upon the subject agree that the compass was known long before that period to the Indians and Chinese; nay, in Bowles's Spirit of Maritime Discovery, it is attempted to be proved, by much "learned argument," that it was known to Noah, and formed part of the equipment of the ark.
The compass is extensively used in surveying and mining, as well as in navigation; but there is in general this difference between them, that marine compasses have a circular card divided into thirty-two parts or points, and attached to the needle; but in those for other purposes the card is fixed to the bottom of the box, which is turned till the needle coincides with the north point of the card. Marine compasses are usually supported" upon a pin of steel or brass, having a fine point, working in a socket of agate let into the centre of the needle or magnetized bar; the pin is fixed into the bottom of a circular box or basin, which is suspended by two pins turning in holes in a ring concentric with the circular box; this ring again is suspended by two pins at right angles to the former, the whole forming a universal joint. Besides the steering compass, there is another one made use of at sea, for the purpose of taking the sun's bearing, in order to ascertain the variation of the compass, or the difference between the true and magnetic north points, as also in taking the bearings of different objects in marine surveying; this is called the azimuth compass.
The card, besides the thirty-two points, is accurately divided at the circumference into 360 degrees, and on the opposite sides of the compass-box are two sights, for taking accurately the bearing of an object, and a stud for stopping the card at the instant the bearing is obtained, so as to allow time for reading off the angle. The circular box is suspended within a square box, which latter is supported upon a pivot (on the top of a three-legged stand), so that the right vanes may be turned to any part of the horizon. Captain Phillips, R.N. has taken out a patent for an improved mode of mounting ship's compasses; the object of which is to prevent those derange ments which sudden concussion occasions to these instruments, by which the card is frequently thrown off its central pivot, as is sometimes the case in stormy weather, or from firing guns, striking of steam-boat paddles, etc. The figure in the following page, Fig. l, gives a vertical section of the improved compass, a is the standard formed of a hollow tube which incloses a long spiral spring, resting upon a bearing at b; to this bearing are fixed two handles cc, by which it may be elevated within the standard, (compressing the spring,) and the bearing may rest at any of the notches or recesses Nos. 1,2,3,4, which are formed in opposite sides of the tube, to receive the handles c c. d is a short solid cylindrical piece, so fitted into the hollow standard as to slide freely within it, and resting upon the spiral spring; e is a cap fixed on to a by a bayonet joint, to prevent the spring from forcing the sliding piece d out of the standard; f is the compass-card, which rests upon the pivot g; this pivot passes through the cross bars l l, and the lower end is formed into a spherical knob, (better seen at h, Fig. 2, which exhibits the same parts upon a larger scale).
This spherical knob rests upon a flat surface of agate, cornelian, or other hard stone i, which lies in a recess upon the top of the piece d, and is kept in its place by the cap k being screwed down upon it; the cap k is made concave in the upper part of it, to fit the spherical knob, so that the latter, resting at bottom upon a hard plane, and above in its hemispherical concavity, can turn freely in any direction. At mm is shown (black) the section of a leaden ring, which surrounds the compass-box, as is shown by the two parallel dotted lines. At n n are two handles fixed to the leaden ring, by which it may be shifted higher or lower on the case, and be fixed at any height at pleasure, by the palls o o dropping into the racks p p, which are screwed to opposite sides of the case, q is the steering, or " lubber's point," which is suspended upon a movable joint r as a centre of motion, and oscillates as a pendulum. This centre being placed so as to slide between two vertical guides, it may be raised or lowered at pleasure, ana is retained in its place by the pressure of a spring in the centre.
The lubber's point should be adjusted so as to be even with the upper edge of the compass, by raising or lowering the centre r, and the bob s should be shifted along the rod until it coincide with the centre of the leaden ring. If the motion of the compass-box, in a vertical direction, should be too violent, it is to be checked by raising the handles c c into one of the notches 1, 2, 3, 4, as may be required by circumstances. The oscillations of the compass-box are regulated by shifting the leaden ring, raising it in blowing weather, and lowering it in fine weather. It frequently happens at sea that the sun is visible, although the horizon cannot be distinguished for fog and haze, in which case it becomes impracticable to take the sun's altitude, in order to determine the ship's place. This circumstance, at all times inconvenient, may, in some situations, be attended with the most fatal consequences. To obviate this difficulty, Lieut. G. Linde-say, R.N. proposes to take two bearings of the sun, as near noon as possible, noting the time elapsed by a good watch, and from these data to compute the sun's meridian altitude in the usual way, which will give the latitude and apparent time at the ship; and this latter, compared with the Greenwich time, will give the longitude.
For taking the sun's bearing he proposes to attach to a common compass, mounted on a pivot at the summit of a tripod, a telescope, turning upon a joint, and a magnifying glass to read off the divisions, as shown in the annexed diagram. It appears to us that the common azimuth compass (with which all ships are or ought to be provided,) will be found much superior to the above contrivance, by which we fear it would . be difficult to observe the bearing with sufficient accuracy.