COMPASS westerly in the Atlantic and Indian oceans, easterly in the Pacific. The vertical plane passing through the longitudinal axis of such a needle is known as the Magnetic Meridian. Following the first chart of lines of equal variation compiled by Halley in 1700, charts of similar type have been published from time to time embodying recent observations and corrected for the secular change, thus providing seamen with values of the variation accurate to about 30' of arc. Possessing these data it is easy to ascertain by observation the effects of the iron in a ship in disturbing the compass, and it will be found for the most part in every vessel that the needle is deflected from the magnetic meridian by a horizontal angle called the Deviation of the compass; in some directions of the ship’s head adding to the known variation of the place, in other directions subtracting from it. Local magnetic disturbance of the needle due to magnetic rocks is observed on land in all parts of the world, and in certain places extends to the land under the sea, affecting the compasses on board the ships passing over it. The general direction of these disturbances in the northern hemisphere is an attraction of the north-seeking end of the needle; in the southern hemisphere, its repulsion. The approaches to Cossack, North Australia; Cape St Francis, Labrador; the coasts of Madagascar and Iceland, are remarkable for such disturbance of the compass. The compass as we know it is the result of the necessities of navigation, which have increased from century to century. It consists of five principal parts—the card, the needles, the bowl, a jewelled cap, and the pivot. The card or “ fly,” formerly made of cardboard, now consists of a disc either of mica covered with paper, or of paper alone, but in all cases the card is divided into points and degrees as shown in Fig. 1.
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paper marked with degrees and points, as in Fig. 1, attached to a frame like that in Fig. 3, where an outer
Fig. 2.—Admiralty Compass Fig. 3.—Thomson’s (Lord Kelvin’s) (Frame and Needles). Compass (Frame and Needles). aluminium ring, A A, is connected by 32 radial silk threads to a central disc of aluminium, in the centre of which is a round hole designed to receive an aluminium cap with a highly-polished sapphire centre worked to the form of an open cone. To direct the card eight short light needles, N N, are suspended by silk threads from the outer ring. The magnetic axis of any system of needles must exactly coincide with the axis passing through the north and south points of the card. Single needles are never used, two being the least number, and these so arranged that the moment of inertia about every diameter of the card shall be the same. The combination of card, needles, and cap is generally termed “ the card ” ; on the continent of Europe it is called the “ rose.” The section of a compass bowl in Fig. 4 shows the mounting of a Thomson card on its pivot, which in common with the pivots of most other compasses is made of brass, tipped with osmium-iridium, which although very hard can be
Pro 4 —Section of Thomson’s Compass Bowl. C, aluminium cap with sapphire centre ; N, N', needles ; P, pivot stem with pivot.
Fig. 1.—Compass Card. The outer margin is divided into degrees with 0 at north and southland 90° at east and west; the 32 points with half and quarter points are seen immediately within the degrees. The north point is marked with a jleur-de-lis, and the principal points, N.E., E., S.E., &c., with their respective names, whilst the intermediate points in the figure have also their names engraved for present information. The arc contained between any two points is 11 15 . The mica card is generally mounted on a brass framework, F F, with a brass cap, C, fitted with a sapphire centre and carrying four magnetized needles, N, N, N, N, as in Fig. 2. The more modern form of card consists of a broad ring of
sharply pointed and does not corrode. Fig. 4 shows the general arrangement of mounting all compass cards in the bowl. In Fig. 5 another form of compass called a liquid or spirit compass is shown partly in section. The card nearly floats in a bowl filled with distilled water, to which 35 per cent, of alcohol is added to prevent freezing; the bowl is hermetically sealed with pure india-rubber, and a corrugated expansion chamber is attached to the bottom to allow for the expansion and contraction of the liquid. The card is a mica disc, either painted as in Fig. 1, or covered with linen upon which the degrees and points are printed, the needles being enclosed in brass. Great steadiness of card under severe, shocks and vibrations, combined with a minimum of friction in the cap and pivot, is obtained with this compass. All compasses are fitted with a gimbal ring to keep the bowl and card level under every circumstance of a ship’s motion in a seaway, the ring being connected with the binnacle or pedestal by means of journals or knife edges. On the inside of every
