looking through the body the wire appears to bisect the bubble, seen by reflection in the mirror.

Use. When the line of sight is directed upon any object and the bubble brought to the center of its tube, or until the wire bisects its reflected image, the slope of the line of sight may be read on the graduated arc, which is divided to degrees and numbered each way from the zero to 60° or 90°. On the double vernier (Fig. 69, p. 61), on the end of the arm the smallest reading is 10.

When the vernier is set at zero, the instrument may be used as a hand level to find points at the same elevation as the eye.

The graduations on the inner edge of the limb are the denominators, unity being the numerators of fractions expressing slopes, as, 1, etc. The graduation in coincidence with the front edge of the arm is read.

Adjustment: To test for index error. Place the instrument on top of a fence-post, sight to the top of another one several hundred feet distant, bring the bubble to the center; and read the vernier. Go to the other post, sight back, bring bubble to center, and read. One-half the difference of the readings will be the index error to be applied to all readings, being careful to observe whether it is to be added to or subtracted from angles of elevation, and vice versa. If the first observation was 2° elevation and the second 3° depression, the index error is to be added to all elevations and subtracted from all depressions.

To make the adjustment. If where it can be done, place it upon a smooth inclined surface, bring the bubble to the center, and read the vernier; reverse it end for end in the same place, bring bubble to center again, and read vernier. If in adjustment, the two readings will be the same; if not, the difference will be the index error. Apply the index error to one of the readings, set the vernier at that reading, place on the inclined surface, and bring the bubble to the center by the screws at the end of the level-tube. Repeat for a test.

SLOPE BOARD (Fig. 207).—Description. A substitute for the clinometer may be arranged on the field drawing board. If one edge is straight, this may be used as the line of sight. On the back of the board construct a graduated semicircle with its diameter parallel to the line of sight, or paste it on a graduated

FIG 207

semi-circular protractor printed or constructed on paper. The graduations shonld be numbered in degrees each way from the center. Then if a plummet or pendulum be suspended from the center of the diameter, and the line of sight be inelined, the reading of the arc opposite the thread or wire, will be the degree of inclination, for the thread retains a vertical position, while the board and attached arc are inclined.

Use. To determine a slope, bring the line of sight parallel to it, with the plummet free of the board, then carefully tilt the board until the thread lies on the arc, secure it with the finger and read off the degrees. To determine a level line, with the thread passing through the 0 mark, stick a pin against one side of the thread, then hold the board up, until the thread just touches the pin, and sight along the edge. In order to bring the upper edge of the board in line with the slope, it may be necessary to stand back above it until the prolongation of the surface strikes the eye, or to kneel or lie down until the eye is on a level with the grass or some object judged to be at the height of the eye when the slope is taken to it.

CHAPTER XVIII.

CONVENTIONAL SIGNS AND SYMBOLS.

The conventional signs and symbols adopted for representing forms and features on a map are such as to suggest, if possible, the objects for which they stand. Ordinarily no effort will be made to show lights and shadows on military maps, but when they are shown the light is supposed to come from the upper left-hand corner at an angle of 45° with the horizontal.

WITHOUT COLORS.-Forests of deciduous trees, except oaks, are represented by signs suggesting irregular projections of trees with bushy tops. In representing oaks, the loops have their points out instead of in as shown.

Perennial trees, as pines, firs, etc., are represented by stars. Uncultivated land, which is neither cleared nor forest, is represented by the signs for small trees, grass, and rocks if present.

Meadows or cleared land by signs suggesting tufts of grass, composed of 5 or 7 short lines, the middle one being longest; the bases straight and parallel to the bottom of the map; the tufts evenly but not too thickly distributed and not in lines; dots may be added to produce pleasing appearance.

Cultivated land by signs suggesting furrows, consisting of alternate broken and dotted lines, the breaks short but not ●pposite each other, the dots fine and close together.

Orchards by regularly distributed trees, sometimes shaded. Marsh by parallel lines and tufts of grass; if of fresh water, the lines are broken and filled in with the tufts; if of salt water, the lines are continuous and the tufts made over them.

Ponds by equidistant lines parallel to the bottom of the map, suggesting still water.

Streams and large bodies of water by lines suggesting waves along the shore. The high- and low-water levels by distinct shore-lines. If streams are not large enough to be water-lined, they are represented by a full sinuous line.

Dry runs by dashes and three dots between.

Sand and gravel by dots.

Mud by short dashes.

Buildings on small scales are usually shown as rectangular blocks; on large scales, by the shape of the building in outline, and filled with fine diagonal lines, the outline for wooden buildings being very fine, for brick heavy, and for stone very heavy.

Streets of towns and villages should be shown as they are. Crops. Although signs are often employed to represent different crops, features, etc., it is not safe to trust their doing so without the name attached. Their employment is more for pictorial effect than otherwise.

Arroyos or gullies have their outlines sharply marked, and hatching lines are added to represent the slopes or wash of the earh.

Embankments or fills have the highest outlines sharply marked and hatching lines for the slopes or fall of earth.

Cuts have the cutting or highest line sharply marked and hatching lines for the slopes. The cuts up to the mouth of tunnels are shown. The tunnel itself is shown by broken lines.

WITH COLORS.*-The use of colors on hastily executed military maps will ordinarily be limited to a very few in number (four or five), and they will be applied with colored pencils which can be carried in the field. The forms of the various conventional signs are similar to those already described.

*For a complete description of the use of water-colors on maps, and how to apply them, the student is referred to "Topographical Drawing and Sketching," by 1st Lieut. Henry A. Reed, U, S. A.