any other compass whereby the ship's course is set, is that the center of the compass card and the lubber line a, Fig. 13, shall coincide with the line of keel, or fore-and-aft line xy, of the ship, or a line parallel thereto. Then the point on the compass card, which 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 movement of the ship's bow, either to the right or to the left, will be noticed at once by the helmsman by the motion of the lubber line in relation to the compass points.

49. The standard, or azimuth compass, is aptly called the "navigating compass." By it the ship's courses should be set and all bearings taken for ascertaining the ship's position as well as the bearings of the sun. To this end it is placed where as large an all-around view of the horizon as possible can be obtained, and it should be as free as possible from masses of iron or steel. This compass should be of the best possible workmanship.

FIG. 13

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.

50. A compass placed on a wooden pillar or post is called a pole compass. If a ship is fitted with a pole, mast (when placed on 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 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.

51. The Azimuth Circle.-To facilitate taking bearings, an instrument called the azimuth circle is applied to the standard compass. This instrument consists of a com

FIG. 14

n

position 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 verniers 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 vanes c, d, Fig. 14, are exactly opposite each other and in line with the center of the card; they are hinged so as to turn down when not in use. Each of them contains a vertical slit, the one 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, that accurately divides it, and to which is hinged 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 and the method of obtaining the bearing of a celestial body, as the sun s, above the horizon. The objects 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. 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.

THE LIQUID COMPASS

52. 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 those compasses whose bowls are 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 Ritchie Liquid Compass.-The standard for excellency in liquid compasses has been attained by the Ritchie liquid compass, adopted by the United States Navy and growing in favor in the merchant service.

This compass, shown in Fig. 15, consists of a skeleton card 7 inches in diameter, made of tinned brass, resting on a pivot c in liquid, with provisions for two pairs of magnets. The card is mounted in a bowl a a of cast bronze, the glass cover of which is closely packed with rubber, entirely preventing the evaporation or leakage of the liquid, which completely fills the bowl. Beneath the bowl is a metallic self-adjusting expansion chamber e of elastic metal, by

means of which the bowl is kept constantly full of liquid without the show of bubbles or the development of undue pressure caused by the change of volume of the liquid due to changes of temperature. The rim of the compass bowl is made rigid and its outer edge turned strictly to gauge to receive the azimuth circle. The lubber's line is a fine line

and divided into half-degrees, with legible figures at each 3 degrees for use in reading bearings by an azimuth circle, or in laying the course to degrees.

The card is further provided with a concentric spheroidal air chamber AC, that buoys the weight of the card and magnets, allowing a pressure of between 60 and 90 grains on the pivot at 60° F. or to within 18 grains at 13°. The air vessel contains a hollow cone n that is open at its lower end, and is provided with the pivot bearing, or cap, containing a sapphire that rests upon the pivot and so supports the card.

The cap is provided with two small screws for accurate adjustment in centering the card.

The pivot is fastened to the center of the bottom of the bowl by a flanged plate dd and screws. Through this plate. and the bottom of the bowl are two small holes that communicate with the expansion chamber and admit of a circulation of the liquid between it and the bowl. The point of the pivot is of bell metal and is moderately sharp; h is a hole in the side through which the liquid is poured.

The weight of the card, complete, in air is about 3,060 grains, of which 1,600 grains is in the steel of the magnets.

55. The magnet system of the card consists of four cylindrical bundles of steel wires b, b', b', b'', each wire being .06 inch in diameter, laid side by side and magnetized as a bundle between the poles of a powerful electromagnet; they are afterwards placed in a cylindrical case, sealed, and secured to the card.

Two of the magnets b', b' are each 54 inches long, and are placed parallel to the north and south diameter of the card and on the chords of 15 degrees, nearly, of a circle passing through their extremities; these magnets pass through the air vessel to which they are soldered and are further secured to the bottom of the ring of the card.

The other two magnets b, b'" of the system are each 4 inches long, and are placed parallel to the longer magnets on the chords of 45 degrees, nearly, of a circle passing through their extremities; they are secured to the bottom of the ring of the card.

Actual contact is secured at all times between the cap and the pivot, the friction being diminished by the buoyancy of the card and the smooth sapphire bearing of best stone and workmanship.

The compass is mounted on gimbals in its binnacle, in which it has an easy motion, the leaden weight in the false bottom of the bowl assisting to keep the card horizontal during the rolling and pitching of a vessel at sea.*

*Abstract from "Navigation Notes, U. S. Naval Academy."