CHAPTER III.
THE MAGNETIC COMPASS AS A PART OF A SURVEYING INSTRUMENT OR SEPARATELY—BROAD AND EDGE-BAR NEEDLES—MANUFACTURE OF THE NEEDLE—MAGNETISATION—SUSPENSION—DIP AND ADJUSTMENT—LIFTING—INCLINATION—DECLINATION—VARIATION—CORRECTION—COMPASS-BOXES—DESCRIPTION OF COMPASSES—RING COMPASSES—TROUGH COMPASSES—PRISMATIC COMPASSES—STAND—SURVEYING WITH COMPASS—POCKET COMPASSES.
116.—The Magnetic Needle, which forms part of a great many surveying instruments, is made of the form adapted to the special purposes of the instrument in which it is placed. There are two prevailing forms commonly in use—one in which the needle is made pointed at one or both ends to read directly upon a divided circle fixed upon the instrument, and the other form in which it is made to carry and to direct a divided circle by its magnetic force.
Fig. 26.—Broad needle.
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Fig. 27.—Edge-bar needle.
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The magnetism which gives directive force to the needle has been found by experiment to reside in every separate part of the magnet, that is, it is assumed to be a molecular force. Therefore, it would not appear to be very important, within certain limits, of what form the magnetic needle is made, and this is found by experiment to be to a large extent true. The only important conditions appear to be that the needle shall be of such form that the inducing magnet, to be described, arts. 120–123, which is used for magnetising may be brought into contact upon every part of its surface, and that the molecular continuity of the parts should mutually support the general directive influence of the magnetism longitudinally in parallel lines.