Page:Popular Science Monthly Volume 34.djvu/309

Rh —To relate what is known or conjectured regarding the origin, history, and development of the compass would not be pertinent to this article, and, besides, such information is readily accessible in any encyclopædia; to impart in a general way a knowledge of its construction is more to the point, especially in view of the object of this paper, which is to treat of the behavior of the instrument in an iron ship; and this kind of knowledge is neither easily obtained nor generally free enough from technical terms to be readily intelligible to the non-professional reader.

Like almost every other instrument, the compass has representatives of many a type; to explain the mechanical and magnetical principles of construction, however, in their general application, it is necessary to have reference to some particular type, and for this purpose I shall select the one that in my opinion is the most trustworthy for steering a ship from her port of departure to her haven of destination—the Ritchie liquid compass. This is an American invention, Mr. E. S. Ritchie, of Boston, having many years ago taken out a patent for a liquid compass to be used at sea. From time to time it has been improved, until to-day, in the seven-and-a-half-inch compass supplied our navy, is probably realized the most accurate and complete instrument afloat. So generally has the excellent workmanship of the manufacturer been appreciated, that his compasses now guide the ships of many a nation in every sea. In the wheel-house of the latest large floating structure—the British steamship City of New York—will be found a Ritchie liquid compass.

The compass and its several parts are represented in Figs, 4 to 7, and the reference-letters in every instance pertain to the same parts: E is a copper bowl, with two short arms, D, D (one only being visible), which rest in the grooves of an outer ring,jB; this, in turn, has two short arms, C, C, which repose in the sockets of the binnacle, as the case for holding the compass on a ship is called. This method of suspension, termed gimbals, allows the bowl to swing freely in two planes, so that really it partakes but little of the rolling and pitching motion of the ship. A slender brass spindle known as the pivot (P, Figs. 4 and 7) is screwed into the bottom of the bowl, and on it rests the card. The bowl is filled with a liquid composed of nearly equal parts of pure alcohol and distilled water, and is hermetically sealed with a plane glass cover, which permits the card to be distinctly seen without distortion. The card (Fig. 6) consists of an outer rim (M), a central bulb (K), and four tubes, H, H, H, H—all made of very thin sheet-brass. The rim has double curvature—circular around the pivot, and semi-cylindrical from the inner to the outer edge, as shown in section in Fig. 5. The card is painted white, and has