56. The bearing plate, or pelorus, shown in Fig. 16, resembles a compass with azimuth circle attached, but it has no magnets nor bowl. In the center revolves a metal bar a a that is furnished with sight vanes by which bearings are taken. Below and moving on the same pivot is a dumb compass card, of metal, marked to quarter points and having a rim that is divided into degrees, and that is marked from 0° on each side to 180°. These are read from the verniers at each end of the bar a a within the sight vanes. The card and bar can be clamped at pleasure. The whole is contained in a square box suspended in gimbals, and is kept horizontal by a weight underneath. Screws fitted to one of the supporting arms adjust the lubber's line exactly to the ship's head. Several other bearing plates, more simple in construction. and general handiness, are now in use. 57. In some vessels there are fastened at convenient places, preferably on rails where a good view can be had, plates containing two holes or sockets (the center line of which coincides with the fore-and-aft line of the ship) wherein two corresponding pins, or pegs, attached to the underside of a bearing plate can be fitted, thus giving the instrument its correct position. The pelorus is an instrument extensively used in coast navigation. How to use either of these instruments in taking bearings and to place the ship's head in any desired magnetic direction is described in pamphlets entitled "Directions for Use" accompanying each instrument. Examples illustrating the use of the pelorus will be given in connection with the subject on nautical astronomy. VARIATION, DEVIATION, MAGNETIC DIP MAGNETISM 58. Magnetism is the name given to the phenomenon displayed by magnets, which are of two kinds, natural and artificial. 59. By natural magnets are meant the ore of iron, familiarly known as "lodestone," which possesses the power of attracting small pieces of iron and steel. 60. Artificial magnets are bars of steel or iron to which magnetism has been communicated or imparted by artificial means. Thus, if a bar of slightly tempered steel be held in a vertical position and struck several blows with, for instance, a wooden mallet, it will acquire the property of attracting iron filings at its two extremities, and, hence, becomes an artificial magnet. Magnetism may also be imparted from one magnetized bar of steel to a large number of similar non-magnetic bars by rubbing one-half the length of the latter with the former; the magnetism thus imparted to the bars is then called induced magnetism, or magnetism by induction. 61. The property of attracting iron is not manifested equally at all points of the surface of a magnet. For instance, an ordinary bar magnet when plunged into iron filings does not become enveloped uniformly with the filings, but, instead, the filings arrange themselves around the ends of the bar in feathery tufts that steadily grow smaller as the middle of the bar is approached leaving that portion bare, as shown in Fig. 17. The points and p', around which the filings apparently concentrate, are called the poles; the middle portion, where there is no visible magnetic force, and to which the filings refuse to adhere, is called the neutral zone; and the line pp connecting the two poles is called the axis of magnetism. 62. A magnetized needle when resting upon a fine point so as to turn freely in a horizontal plane will, when not in the vicinity of other magnets or magnetized iron, always come to rest with one end pointing toward the north and the other toward the south. The end pointing northwards is, therefore, called the northseeking pole, and the opposite end, the south-seeking pole. This tendency, called polarity, applies to all magnets. FIG. 17 63. Law of Magnetic Attraction and Repulsion. When two magnetized bars, or needles, are brought close together, two poles attract and two poles repel each other. Thus, the north-seeking end of a magnetic needle will repel the north-seeking end of another needle while it will attract the south-seeking end. Hence, the following law for attraction and repulsion may be enunciated: Poles of contrary names attract each other, while poles of the same name repel each other. 64. Magnetic Property of the Earth.-If a magnetized needle be suspended in such a manner as to have perfect freedom of motion, it will assume a north-and-south direction. This fact, that a suspended needle will take up a fixed position, has led to the theory that the earth itself is a magnet having its north and south magnetic poles in the neighborhood of the geographical poles, and that the magnetic needle turns to these as to the poles of an original magnet in accordance with the law just given. 65. Considering the earth as a magnet having one pole in the northern and the other in the southern hemisphere, and remembering that poles attracting each other are of contrary names, it follows that the magnetic pole of the earth, which is situated in the northern hemisphere, is contrary to that end of the magnetized needle that points to the north; in other words, the north-seeking end of the needle is, in reality, its south pole. From this fact considerable confusion has arisen, but popular usage, however, calls the northseeking end the north pole, and that which points to the south, the south pole. Makers of magnets usually mark the north-seeking pole with the letter N. In many works on magnetism the north-seeking pole is referred to as the red or positive(+) pole, and the other, or the south-seeking pole, as the blue or negative(-) pole. 66. The Magnetic Meridian.—The direction that the horizontal needle assumes when uninfluenced by local attraction is called the magnetic meridian. In general, the magnetic meridian does not coincide with the geographical meridian, and there are but few places on the earth at which the compass needle points to the true north. VARIATION 67. The angle that the magnetic meridian makes with the geographical meridian, or, what is the same thing, the angle that the direction of the suspended needle makes with the true meridian, is called the variation or declination of the magnetic needle, or simply the variation of the compass. Thus, if NS, Fig. 18, represents the direction of the geographical meridian and n the north point of the magnetic needle, the angle Non is the variation of the compass. The variation is named westerly if the north-seeking end W N FIG. 18 66 E of the needle is deflected to the west of the true meridian; when the north-seeking end is deflected to the east of the true meridian, the variation is named easterly. 68. The variation of the compass is different at different places on the earth's surface and at a given place it undergoes a gradual change from year to year. At such places where the magnetic meridian coincides with the true meridian, the variation is said to be zero, and lines connecting these places are called lines of no variation," or agonic lines. In Fig. 19 the lines of no variation are represented by heavy thick lines, westerly variation by continuous fine lines, and easterly variation by dotted lines. A glance at this chart tells us that the variation is westerly nearly all over the North Atlantic Ocean, increasing in amount toward the north, and that the variation is easterly in the greater part of the Pacific Ocean. Lines connecting points of equal variation are called isogonic lines. 69. Magnetic variation undergoes a progressive change in amount, and, after long periods, changes of direction, or secular changes, take place; in other words, the variation vibrates between certain limits. The line of no variation which now crosses the American continent passed through |