Tile deviation of the compass is produced. as has already been stated, by the action of the magnetism of the ship in which the compass is mounted. In the days of wooden ships the deviation was usually very small, even in steam ers. But steel and iron hulls produce important deviations in well-placed compasses, and ex cessive deviations in those badly located: it is possible. indeed. so to place a compass that the needle will point to a particular part of the ship. without regard to her heading. In placing compasses. clue regard must then he paid to the magnetic forces of the ship. as well as to the convenience of the navigator and helmsman. standard compasses are commonly placed at some distance above the upper deck and abaft the middle of the ship. The steering compasses are placed beside the wheel, or steering appa ratus. I )they compasses are found in large ships.
The deviation of a compass varies as the ship turns about, or in azimuth. A portion of the hull is permanently- magnetized and acts as a permanent magnet: the rest is in a variable state of magnetization. depending upon the direction in which the ship heads and the mag netic latitude. As it is important that the devia tions in all directions should be known. in order to steer a correct course, they are determined by swinging ship. This is accomplished by suc cessively pointing the ship in the various direc tions (usually every two points) and comparing the bearing of the sun or a distant object with its true bearing obtained by computation, or from prepared tables. This gives the compass error. Applying the variation to the result, we ob tain the deviation on each point on which bear ings were taken. A table of deviations is a necessary adjunct to every compass. When the deviations are considerable, as is almost invari bly the case with steering compasses. and usually with standards, it is customary to cor rect them by reducing the deviation. The char acter and extent of the correction or compensa tion may be ascertained by magnetic observa tions on the ship and on shore: from which follow elaborate computations upon the results of which depend the placing of the correctors. This method involves an investigation of the whole theory of magnetization of ships. A simpler one now rapidly coming into favor is called the rectangular method of compensating. The deviation is determined upon adjacent car dinal points, as (magnetic) north and east. The ship is headed on north, and kept on that head ing by using another compass, and a number of small magnets slipped into holes in a block or ease under the compass. These magnets point fore and aft. and are inserted until a sufficient number have been put in to bring the compass to point to magnetic north. The same process is repeated when the ship is headed east. but the
magnets now put in are laid athwartship. Next the ship is headed northeast (magnetic.). and the soft iron spherical correctors are moved out (i.e. away from the compass) until the needle points correctly. This completes the operation. unless a Flinders bar is used: this consists of a soft iron vertical bar, placed to counteract the induced magnetism in soft vertical iron, and is placed forward or abaft the compass, as found necessary. The ship must then be swung for rcsidun/s; that is, turned and headed on the different points (or every two points), and the small residual errors remaining, after compensa tion ascertained. These may be reduced by re correcting. but are usually too small to render that necessary: a table of these deviations is. however, made out for use in laying the ship's course. In addition to the deviation of the compass produced in the manner already de scribed, further deviations are caused by the rolling. This is corrected by a vertical magnet, placed below the axis of the compass and at such a distance as will correct a deviation pro duced by the inclination of the ship.
The compass-card is divided into 32 points. Of these. four are called the cardinal points, north, east, south, and west (abbreviated N., E., , and W.) : and four are the intercardinal points—northeast, southeast, soutImest. and northwest (abbreviated S.E., S.W., and NAV.). The points half way between a cardinal and intercardinal point has a name compounded of the two. the cardinal point being given first thus, half way between N. and N.E. is north northeast (N.N.E.), and half way between east and northeast is east-northeast (E \.E.). The point on each side of a cardinal or intercardinal point is designated by first giving the name of the primary point, then the word by and the name of the adjacent cardinal point. Thus, the point to the cast of north is north by east (N. by E.) ; to the east of northeast is northeast b east Y.E.( by E.) ; similarly, we have east by north, south by west, etc. Snmller angles than point, are given in half-points, quarter-points, and eighth-points. These are, in addition to the full points, reckoned from north and from south toward cast and west : remembering, how ever, that from the cardinal and intereardinal points for one point each way the fractional parts of a point are reckoned toward the adjacent cardinal point. 'nil,: gives the points of the compass as follows: To recite these points. half-points. quarter points, and eighth-points in the proper order is eallcd boxing the compass. and to do so correctly, and to understand the taking of bearings, etc., arc among the requirements of a good helmsman or qua rterma e r ( q.v.) .