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ment facilitates very much the taking of star azimuths, by throwing (as in the camera lucida) an image of the star upon the graduated circle of the compass card, or, more properly speaking, on a virtual image of this scale at an infinite distance, as seen through a convex lens. It is easy at sea, even when the ship has considerable motion, to read the position of a star accurately to a small fraction of the white space on the card between two degree divisions on which its image is seen. The focal length of the convex lens is a little greater than the radius of the circle, and thus for objects on the horizon, or at any altitude not exceeding 30°, no further adjustment of the azimuth appliance than to bring the object fairly into the field of view, is required.

Another improvement which allows the compass-bowl always to keep a truly level position, is the use of knife edges instead of journals for supporting the gimbals. For taking azimuths accurately it is most important that the compass-bowl should be truly level, but when a compass is supported on journals the friction between the journals and supports is very injurious in preventing it from keeping the level position. When knife edges are used for the axes of the gimbals, the bowl takes a level position with almost perfect accuracy. A hemispherical hollow space below the glazed case in which the compass works, nearly filled with castor oil, serves to calm the vibrations of the bowl, which, without this appliance, would sometimes be troublesome owing to the want of friction in the knife edge bearings. The gimbals are suspended from a wire grummet ring, still further to reduce vibration.

DESCRIPTION OF THE FLINDERS-BAR.

The Flinders-bar supplied with Sir William Thomson's Compass, is a round bar of soft iron, 3 inches in diameter, and of whatever length from 6 to 24 inches, is found to be proper for the actual position of the compass in any particular ship. To make up the proper length, it is supplied in pieces of 12, 6, 3, and 14 inches, also two pieces of of an inch. In making up the proper length, the longest piece should be uppermost, and the others below in the order of their lengths. The weight of the bar is supported on a wooden column resting on a bracket, fixed near the foot of the binnacle; this wooden column being cut to such a length, or so made up of pieces, as to give the proper height to the upper end of the iron bar. The compound column of iron and wood is kept in position and protected from rain and spray by a brass tube with upper end closed.

The main object of the Flinders-bar is to counterbalance the component of the ship's horizontal force on the compass, which is due to magnetism induced by the vertical component of the terrestrial magnetic force. Hence, in all ordinary cases, the ship's iron being symmetrical on the two sides of the fore-and-aft midship vertical plane, and the compass being placed in this plane, the Flindersbar must be placed in it also, exactly in the middle of the front or back of the binnacle. The Flinders-bar essentially corrects, wholly and permanently, the Heeling error, which has maximum value on the East and West courses, and vanishes on the North and South courses. This error is due to horizontal force produced by magnetism, induced by force perpendicular to the deck. It is towards the lower side of the ship on a West course, if towards the upper side on an East course; or is towards the upper side on a West course, if towards the lower side on an East course.

A subordinate object of the Flinders-bar is to partially correct the Heeling error, which has equal maximum value when the ship's head is North or South, and is zero when the ship's head is East or West, by partially counterbalancing the component force on the compass, perpendicular to the ship's deck, exerted by that part of the ship's magnetism which is induced by the vertical component of the earth's magnetic force. When it is not desired to make this contribution to the heeling correction by the Flinders-bar, it should be placed with its top about two inches above the level of the needles of the compass card. A large part of this error depends on the upper parts of the ship's iron having on the whole a polarity, which, for a ship built in the Northern Magnetic Hemisphere, and continuing in the same Hemisphere, is the polarity (blue) of a true North Pole, while the bottom of the ship and keel have, on the whole, a polarity of the opposite kind (red). The remainder is due to inductive magnetization, by the vertical magnetic force of the earth acting on iron deck beams. or iron decks, when inclined to the horizon by the ship heeling over to either side. The combined effect of these influences is to draw the North point of the compass card towards whichever side of the ship is the highest as she heels over, if she is on any other course than due East or West. It is to be corrected at sea, by placing the HEELING MAGNET with its true South pole (red) up, and true North pole (blue) down, in the central tube of the binnacle below the middle of the compass, and sliding it up or down till it is found that changes of the ship's inclination, without changes of course, produce no sensible disturbance on the compass.

This adjustment should not be attempted unless the ship's course is within 60° of either North or South; with this limitation it may be usefully performed at any time when good bearings, whether of sun, moon or stars,

can be had. If when the ship heels steadily over to either side, the North point of the compass shows a deviation towards the upper side of the ship, the HEELING MAGNET if red end up is to be moved up a little, or if blue end up to be moved down. If, on the contrary, the North point of the compass shows a deviation towards the lower side of the ship, the HEELING MAGNET must be moved downwards if its red end is up, or upwards if its blue end is up. If the ship crosses the magnetic equator, it may be found necessary to reverse the HEELING magnet.

A Clinometer (little pendulum) attached to the binnacle facilitates this adjustment, by allowing the amount of every variation of the ship's heeling inclination to be seen along with the deviation of the compass produced by it.

To understand the action of the Flinders-bar, suppose the ship to be anywhere in the northern magnetic hemisphere, the vertical force there is such as to pull the red end or pole of a magnetized needle downwards, and to repel the blue end upwards. It also has the effect of inducing magnetism in any mass of iron, so as to give it a transient magnetic quality marked with blue on the upper side or end, and red on the lower side or end. Thus, in the northern magnetic hemisphere, the Flindersbar is transiently magnetized by the earth's vertical force in such manner that it acts like a great bar magnet, with its upper end blue and its lower end red. At the magnetic equator it loses its magnetism, and in the southern magnetic hemisphere it acquires magnetism in the opposite direction to that which it had in the northern hemisphere; so that its upper end becomes red and its lower end blue. As the ship moves from one hemisphere across the magnetic equator to the other, the magnetism of the Flindersbar gradually diminishes to zero, and then increases gradually in the contrary direction.

The object to be attained in applying it to the binnacle is, that, with this gradual change of its magnetism, it

shall always as exactly as possible counterbalance the changing part of the force on the compass, due to the part of the ship's magnetization which changes with the gradual change of a vertical component of the terrestrial magnetic force. If this changing part of the ship's disturbing force on the compass is a pull aft in the northern magnetic hemisphere, and a pull forward in the southern magnetic hemisphere, the Flinders-bar must be on the forward side of the binnacle. On the other hand, if the regularly changing part of the ship's force be a pull forward in the northern hemisphere, and aft in the southern hemisphere, the Flinders-bar must be on the after side of the binnacle.

The former is the most frequent case for the Standard compass and the steering compasses of modern merchant steamers in which the conning and steering of the ship is done on a bridge forward of the engines, with considerably more than half the ship behind it. It is almost certain to be the case with an after steering compass, a few feet in advance of the top of the stern post and rudder head, in an iron steamer or sailing ship. The second above-mentioned case is what will generally be found with a compass placed anywhere in the after half of the ship's length, to within six or eight feet of the stern post. Generally it is impossible to ascertain which of the two is the actual case until the ship has made a voyage through regions presenting considerable differences of vertical magnetic force.

The best plan generally is, in first placing the binnacle on the deck, to turn it with the fittings of the Flindersbar forward or aft, according as it is found that the foreand-aft correcting magnets have to be placed with red or blue poles forward or aft. It may be that the experience of a first voyage may show that the binnacle must be turned the other way to get the Flinders-bar into its right position, but the chances of this being necessary are

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