COMPASS (OF., Fr. minims, compass, from ME. eompassys, circle, from Lat. corn.-, together + ',assns. step. from pander. to spread out ). The most important instrument used in the nav igation of a ship. It consists of a number of magnetic needles placed with their axes parallel; a framework supporting the needles, and a circular disk marked in points and de grees, called a compass-card: the pivot on which the framework rests; and the ram pass-bwrl, containing the compass. This latter is, in turn, mounted on gimbals in a binnacle (q.v.) and further protected by a binnacle-co•e•. Com passes are of two general types, dry and liquid. The best-known form of dry compass is that designed by Lord Kelvin (Sir William Thom son, before h is elevation to the peerage). It consists of a central boss and outer rim, both of aluminum connected by numerous radial silk threads. The magnets, eight or more in number. are held in parallelism by threads and supported by other threads from the rim. The latter has a paper cover marked in degrees and points. This compass is very light, and therefore will be very sensitive and yet not have too much throw as the ship moves. Furthermore, the weight, such as it is, being largely in the rim, will tend to check a tendency to liveliness. The dry compass is cheap and reasonably satisfactory in other respects. and is much used. In the United States Navy and in many ships of the merchant service a liquid compass is used. The bowl is filled with alcohol and water. The com pass-needles. in two or more bundles, are sealed in parallel tubes. which form the fi:amework connecting the central boss to the rim. Both rim and boss are hollow, and, like the needle tubes, are of white metal or aluminum alloy. 'flee ma•ldngs of the ordinary compass-card are painted upon the rim in degrees and points. The whole float, consisting of rim, boss, and magnetic tubes, is slightly heavier than the liquid. so that it rests very lightly upon an agate pivot-bearing in the boss, which is supported upon a pivot rising from the bottom of the bowl. The liquid checks the tendency of the compass to jump when the ship has considerable motion, and the vi brations of the machinery. wh ich are apt to disturb dry ciimpasses, have no effect upon a liquid one.
The compa ss-needll. is drawn away from the horizontal plane by the vertical component of the earth's magnetie force by an amount called the dip. or hie/inn/ion. The error of the com
pass, which is the 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 true and magnetic meridians q.v.), which represents the deflecting effect of the earth's, magnetism, and the de/in/ion, or angle, between the magnetic meridian and the needle, which represents the deflecting effeet 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 manner, so that, within limits. its amount at any future time may be predicted. II is also subjeet to slight monthly and daily changes. The daily change consists of a small easterly movement during the morning. reaching a maximum about 7 A.M.; then a somewhat faster westerly movement, the limit of which is reached about 1 P.M.; the return easterly move ment is completed at 9 or 10 Both daily and monthly changes are small and of no im portance in navigation. The average daily range is about 1.3 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 lie due to magnetic storms. See MAGNETISM. 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 isogonic lines; the isogonic 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 /hits. The line of no dip is the magnetic (qualm-. The dip of the needle. like the variation. is constantly changing. but the range and rate of change is less. See DECLINA TION; lxcl.lx.vriox.