163.—The author has made a small pocket magnetic compass, which is represented in the illustration above. The needle is placed in a long box. It reads at its point into a single line when the needle is exactly parallel with the sides of the box. The lid turns up endwise. The needle is lifted by closing the box.
Fig. 45.—The author's under slide for setting off variation.
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164.—In a form of compass similar to the above, the author has added a thin slide to the under side of the box, by means of which the magnetic variation may be adjusted, as shown Fig. 45. This slide moves out just the amount of magnetic variation, the stud S being made concentric for this adjustment. If the slide box be made of ivory a few useful scales may be divided upon it. The compass slips into a light leather case, and is the most portable for its length of needle of any compass made. The edges of the box are used as directing lines, as above described for the square form. The illustrations show a compass made for Great Britain, but a similar instrument is also made universal. In this case the box is a little wider, with the centre of the slide in the middle, so that the magnetic variation can be set off west or east. A rider also on the needle enables it to be balanced in southern latitudes.
Fig. 46.—Barker's luminous compass.
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165.—Barker's Luminous Compass, with floating dial of mother-of-pearl, one-half of this being engraved with black figures and the other half painted black with the figures left white, permits magnetic direction to be observed in the dusk and by moonlight. These compasses, Fig. 46, are much used by travellers. Mr. Francis Barker has also designed a compass in which the needle carries a bar coated with luminous paint.