Page:The New International Encyclopædia 1st ed. v. 05.djvu/278

* COMPASS. 230 COMPASS. rising from the liottom of the bowl. The liquid checks the tendency of the compass to jump wnen the ship has considerable motion, and the vi- brations of the machinery, which are apt to disturb dry compasses, liave no effect upon a liquid one. The compass-needle is drawn away from the horizontal plane by the vertical component of the earth's majnietic force by an amouiit called the dip. or incliinHion. The error of the com- pass, which is tlio result of forces acting in the horizontal plane, is the angle between the direc- tion of the needle and the true, or geographic, meridian. It is made up of the variation, or angle, between the ti-ue and magnetic meridians (q.v. ), which represents the deflecting effect of the earth's magnetism, and the derialion, or angle, between the magnetic meridian and the needle, which represents the deflecting effect of the ship's magnetism. The variation is due to the fact that the lines of magnetic force which affect the needle are parallel to the geographic meridian in only a few places on the earth. The direction of one of 'these lines at any point is the direction of the magnetic meridian there : that is, the direction in which a freely suspended magnet will lie. The variation of the compass at any point is not usually stationary, but changes from year to year, increasing to a certain maximum, decreas- ing to a minimum, and then increasing, in a very regular nuinner, so that, within limits, its amount at any future time may be predicted. I± is also subject to slight monthly and daily changes. The daily change consists of a small easterly movement during the morning, reaching a maximum about 7 A. jr. : then a somewhat faster westerly movement, the limit of w'hich is reached about 1 P.M. ; the return easterly move- ment is completed at 9 or 10 p.m. Both daily and monthly changes are small and of no im- portance in navigation. The average daily range is about 9.3 minutes of arc. Irregular changes of variation, occasionally amounting to 3 or 4 minutes of arc in a few minutes of time, are of frequent occurrence; they are said to be due to magnetic storms. See M.^gxetism. Terrestrial. Magnetic charts show the variation, dip, etc. The irregularly curved lines connecting the points of equal variation are called lines of equal variation, or isof/onic lines; the isogonie line con- necting points of no variation is called the agonic line. The lines of equal dip are irregular curves surrounding the magnetic pole, and are called isoclinic lines. The line of no dip is the magnetic equator. The dip of the needle, like the variation, is constantly changing, but the range and rate of change is less. See Declix.- Tion : IXCI.IN.VTIOX. Tiie 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, due regard must then be 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. Other compasses are found in large ships. The deviation of a compass varies as the ship turns about, or in azimuth. A portion of the liull is permanently magnetized and acts as a permanent magnet; the rest is in a variable slate 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 (usuailj' every two points) and comparing the bearing of the sun or a distant object with its true bearing obtained by computation, or from prepared tal)k'S. This gives the compass error. Applying the variation to the result, we ob- tain the deriation 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 obsen'a- 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 detennine3 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 case 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 sliip is headed east, but the nuagnets 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 strung for residuals; 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. how'ever, 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, placecl 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.,