162.—Pocket Magnetic Compasses.—The subject of compasses will scarcely be complete without mention of the small pocket compasses which are so useful and universal. Several well-known forms are shown in the next illustration. The square form shown first, Fig. 40, will be found the most useful for very rough sketching. The edges may be sighted for the direction of roads, etc., or the box may be placed against a wall for taking the magnetic direction of a building. In like manner also the compass-box may be laid on a drawing and lines drawn along by the edges of the box to the magnetic directions taken. This in most cases is sufficiently accurate for architectural work, in which the exact direction is not generally thought to be important. Fig. 41 is a French form of compass with step reading level with the upper surface of the needle. Fig. 42 is an old English form with enamelled dial, with lifter under the bow of the handle. Fig. 43 is the same make in a hunter case. In this the lifter rises upon the case being closed.

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.

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.

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.