Fig. 292.—Artificial horizon, mercury.
Fig. 293.—Mercury bottle to the same.
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656.—Artificial Horizon of Mercury, Fig. 292. This instrument consists of an oblong tray of about 6 inches by 3 inches by ¾ inch in depth made of wrought iron. It is covered by a roof with two sloping sides at about 45° to the plane. The sides of the roof are glazed with worked parallel glass fixed by screws at three points. The mercury when out of use is contained in an iron screw-stoppered bottle, Fig. 293. It is poured into the open tray for use, and the tray is afterwards covered by the roof to prevent currents of air disturbing the level of the surface. After use the mercury is poured back into the bottle from the corner of the tray. It should be particularly observed that it is perfectly drained, as any free particles in the case in which all parts of the instrument are packed would be certain to attack the roof, which is made of brass and simply varnished. The instrument is packed in a mahogany case, size 7½ inches by 6 inches by 5 inches; weight, with 1 lb. of mercury, about 4¾ lbs.
657.—The Bottle, Fig. 293, is made of cast iron. It has a screwed plug stopper with a leather collar and a covering cap with a small hole through its apex. To pour out the mercury the cap and stopper are unscrewed, the plug is taken away, and the cover is screwed on again. The mercury then issues from the small hole in the cap. To return the mercury the cap is reversed and screwed upon the bottle. It then forms a funnel. The tray has a covered corner at which there is a small hole. This permits the mercury to be poured into the funnel without splashing. Both plug and cap are then screwed down firmly, and the bottle is placed in a secure fitting in the case.
Fig. 294.—Captain George's artificial horizon.
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658.—Captain George's Artificial Horizon,[48] Fig. 294. This is a great improvement on that last described. The instrument being made entirely of iron there is no risk of getting it injured by escape of the mercury. It is also much more portable and convenient. Two chambers E and M are connected together by a tube through a stem-piece in which there is a strong iron cock at a. The chamber E is cored out and form a bottle into which about 1 lb. of mercury is introduced by removing a screwed stopper at B. The chamber M is an open tray with a cover formed of a piece of parallel glass placed in an iron rim which screws down upon it. A milled-headed screw at C forms an air plug. The cock moves very stiffly by the leverage given by a tommy-pin, shown a′, which is inserted in the hole at a. The chamber E is slightly elevated to cause the mercury to flow from it to M, the cock being turned on at the same time and the air screw C released a little. By the same arrangement, M being raised, the mercury flows back into the bottle for storage.