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rubbed with plumbago or graphite. No oil, grease, or other fatty substances should be used for this purpose.
46. According to the use to which the mariner's compass is applied on board ships, it is called the steering compass or the standard, or azimuth, compass.
47. The steering compass is the compass used by the helmsman, and by which the ship is steered in a certain direction. The requirements of this compass, as well as of any other compass whereby the ship's course is set, is that the center of the compass card and the lubber's point a, Fig. 13, shall coincide with the line of keel or fore-andaft line x y of the ship, or a line parallel thereto. Then the point on the compass card that is in line with the lubber's point a is said to be the direction of the ship’s head,” or if the ship is moving, to be its “compass course.” It is evident that if the compass is sensitive, the slightest move of the ship's bow, either to right or left, will be noticed at once by the helmsman by the motion of the lubber's point in relation to the compass points.
48. The standard, or azimuth, compass is aptly called the ''navigating compass." By it the ship's course should be set, and all bearings taken for ascertaining the ship's
Fig. 13 position as well as the bearing of the sun. To this end it should be placed where a large all-around view of the horizon can be obtained, and where it is as free as possible from masses of iron and steel, and should be of the best possible workmanship.
Since this compass is superior, and by its position least affected by the disturbing elements of the ship, the steering compass, or compasses, should be constantly compared with it, in order to insure steering the correct course.
The Pole Compass. – A compass placed on wooden pillar, or post, is called a pole compass. If a ship
is fitted with a pole, mast (when placed on the mast itself several yards above the deck), or tripod compass, it has been recommended to consider it as a “standard compass,' and the one on the deck, otherwise known by that name, to be called the “navigating compass,” as mast compasses, when properly constructed by experienced makers and well placed, are often very reliable. Their deviation is very small in amount and much more constant than those nearer the hull. On the other hand, on account of their elevated position, they are subjected to more violent disturbances when the ship is laboring in heavy seas and are consequently at such times less reliable. Furthermore, the pivot and cap of a pole compass should be examined frequently as they are, through the more constant motion of the card, likely to wear out sooner than those of a compass placed on deck.
The pole bearing the compass must be prevented from turning one way or another through changes of temperature or other causes, and should be made of teak and painted white, so as to be least affected by the heat of the sun.
50. The azimuth circle is an instrument to facilitate taking bearings, and is usually made so that it can be affixed
to the standard compass.
The instrument consists of a composition ring turned true to fit over the compass bowl; its center is coincident with the center of the compass, carrying with it sight vanes, mirror, prism, and vernier for reading the graduations of the circle. The circle
is graduated to degrees, and these are subdivided into divisions of 15, 20, or 30 minutes each.
The sight vanesc and d, Fig. 14, are exactly opposite each
other and in line with the center of the card; they are hinged so as to be turned down when not in use. In each of them is a vertical slit, the slit in c, which is for the eye, being narrower than the one in d. The larger vane d is fitted with a very fine thread, or hair, stretched along its center, accurately dividing the slit; the vane is also provided with a reflecting mirror r. By sighting through the two vanes an accurate bearing of the sun or other object can be obtained. Fig. 14 shows the prism h and the method of obtaining the bearing of a celestial body, as the sun s, above the horizon. The object s is reflected in the mirror, or reflector, r, and by the divisions on the compass card n n being reflected to h and from there to the eye, the observer is enabled to read off the bearing at once. Usually the lower side of the prism is convex for the purpose of magnifying the graduations on the card.
51. There are various other forms of azimuth attachments, notably Lord Kelvin's and Ritchie's improved azimuth circle now in use in the United States Navy, but the application and principles of all are similar. The azimuth circle is usually enclosed in a wooden case, or box, when not in use, and is fitted for use with a tripod.
52. The Liquid Compass. - The liquid compass is simply an ordinary compass, the bowl of which is filled with liquid instead of air. It was introduced to overcome the disturbances of the card caused by the sometimes violent motion of small vessels and boats. It is now in general use, and by mariners the world over is considered far superior to the dry compass,” or such compass the bowl of which is filled with air. When well made it is a very efficient instrument, especially adapted to stand the continuous vibrations from the screw propeller, as well as the shock of heavy gun firing.
53. The standard for excellency in this class of compasses has been attained by the Ritchie liquid compass, which has been adopted by the United States Navy, and is growing in favor in the merchant service. The distinctive
feature of this compass is the attachment to the compass card of an air chamber by which almost the entire weight of the card is supported by the buoyancy of the liquid, thus reducing the friction and the wear on the pivot to a very small amount and increasing the sensitiveness of the compass. The liquid is composed of alcohol and distilled water in such proportion that it will remain liquid; that is, it will not freeze at 20° below zero.
54. The bearing plate, or pelorus, shown in Fig. 15, 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 and a reflecting mirror by which bearings are taken in precisely the same manner as with the azimuth circle.
Below, and moving on the same pivot, is a dumb compass card of metal, marked to quarter points and having a rim
divided into degrees marked from 0° on each side to 180°. These are read from the verniers at each end of the bar within the sight vanes, and also at zero on the ring as desired. The card and bar can be clamped at pleasure.
The whole is contained in a square box suspended in gimbals and kept horizontal by a weight underneath. Screws fitted to one of the supporting arms adjust the lubber's point exactly to the ship's head.
55. Several other bearing plates, more simple in construction and general handiness, are now in use.
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 under side 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 azimuth determinations.
We shall now consider the disturbing forces affecting the compass and the means used to counteract or minimize their effect.
VARIATION, DEVIATION, MAGNETIC DIP
56. Magnetism is the name given the phenomena displayed by magnets, which are of two kinds, natural and artificial.
57. By a natural magnet is meant the ore of iron, familiarly known as “lodestone," which possesses the power of attracting small pieces of iron and steel.