(1) The tracing cloth is first fixed by means of the rollers over the board. (2) A magnetic north line is then ruled across the paper and passing immediately over the centre of the compass visible under the paper. (3) The operator then, holding the board by the handle underneath, proceeds to make his map and first brings the magnetic north line immediately over the top of and aligned with the compass needle. (4) He then from some point, marked as his starting point on the paper, proceeds to take sights to any objects he may wish to delineate on his map. These sights are taken by means of the alidade fixed above the board. The sighting rule is pushed along a slotted bar, which itself slides along a bar at the edge of a board until the edge of the alidade is against the starting point and is sighted on the object aimed at. The object is sighted in a small mirror fixed at the end of the alidade and aligned with the point at its other end. The alidade is then clamped into position with the thumb-screw on the slotted bar and the operator draws his ray corresponding with the direction of the object aimed at. The distance is then marked off on the scale on the alidade. (5) When the operator moves to the next station no back-sight is necessary. He at once puts the board into its relative correct position by merely revolving it until the magnetic north line is again lying over the top of and aligned with the compass needle and he then proceeds to take all necessary sights at that station.
Fig. 329.—Cavalry sketching case.
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713.—Cavalry Sketching Case.—This forms a very convenient exploring sketching board, permitting sketches to be made on horseback while en route. The pattern shown in Fig. 329 is that of Captain W. Vernier. It consists of a small board 9¼ inches by 7½ inches, at two sides of which there are small rollers to hold paper 7 inches wide and from 3 feet to 6 feet in length, according to its thickness. Two stout indiarubber bands, which hold a small straight-edge to scale in any position on the paper with sufficient firmness to be able to draw a line against it, are passed over the board. A small compass on one side of the board indicates direction. After one sketch is made, a new part of the paper is rolled forward.
714.—Camera Lucida—Optical Compass.—In new countries where landmarks are not clear a sketch of the general aspect of the country will make the points of triangulation more clear. Where the plane table is not used these sketches may be made with accuracy as to positions by the use of the photograph camera, the camera lucida, or points of observation may be taken in correct bearing by the optical compasses. These latter instruments are described in the author's Treatise on Drawing Instruments, seventh edition.
CHAPTER XVI.
INSTRUMENTS FOR MEASURING LAND AND CIVIL WORKS DIRECTLY—CHAINS—VARIOUS TELLERS—STANDARD CHAINS—ARROWS—DROP ARROW—VICE FOR ADJUSTING CHAIN—CAINK'S RULE FOR INCLINES—STEEL BANDS—WIRE LAND MEASURES—COMPENSATION SYSTEMS—LINEN TAPES—OFFSET RODS—PINE STANDARD RODS—RODS WITH IRON CORE—BEAM COMPASS RODS—COINCIDENT MEASUREMENTS—COMPENSATED RODS—BASE LINE APPARATUS—COAST SURVEY LINES—PERAMBULATOR—PEDOMETER—PASSOMETER—SOUNDING CHAINS—SOUNDING LINES—TELEMETERS—HAND RODS—RULES.