Plotting the notes trains both the hand and the eye in the correct graphical representation of them. The actual instrumental measurement of distances in topographic surveying will assist one in estimating them in sketching. The practice in actually tracing contours with instruments, by which the shape of hill features, etc., is made evident, educates and assists in judging them by eye.

From the constructive point of view, a map is a sketch, corrected by locations. The work of making locations is geometric, that of sketching is artistic. This definition is applicable to all maps, whatever their quality or character. However numerous the locations may be, they form no part of the map itself, but serve only to correct the sketch, while the sketch supplies all the material of the map. The correctness of the map depends upon four elements: 1st, the accuracy of location; 2d, the number of locations per square inch of the map; 3d, their distribution; 4th, the quality of the sketching. The first element depends upon the economy of the work; the second and third upon the character of the country; the fourth upon the artistic ability of the topographer; this latter is the most important, and most difficult to meet.

The immediate object then in topographic surveying is to determine the locations of such a number of points on the earth's surface as will enable one to make a topographic map representing it.

The location of a point in a plane is known when its distance and direction from one or more given points of that plane are determined.

The location of a point anywhere is known when its horizontal distance and direction from, and its elevation above or depression below, one or more given points are determined.

The location of a straight line is known when its extreme points are located.

The distance of a point from a given point is determined by measuring the length of the horizontal projection of the straight line adjoining them.

The direction of a point from a given point is determined by measuring the horizontal angle between the straight line joining the points and a line of known direction through the given point.

The elevation of a point above or depression below a given point is determined by measuring the vertical distance between the level surfaces which contain them.

CHAPTER II.

DRAWING MATERIALS, INSTRUMENTS, AND

THEIR USE.

DRAWING PAPERS.-The selection of the proper kinds of drawing papers very materially assists one in producing good work. Good drawing paper should be strong, be of uniform thickness and surface, stretch evenly, neither repel nor absorb liquids, admit of considerable erasing without detriment to its surface, should not become either brittle or discolored by reasonable exposure and age, and should not buckle when stretched or when inks or colors are applied to it.

Hot Pressed paper is used for fine line drawings. Not Hot Pressed for water colors. Rough for bold drawings and sketches.

Drawing paper may be obtained mounted on muslin.

Tracing Paper and Vellum Cloth are very thin and transparent, much used in copying maps and drawings, and in making blue prints. They are made in both sheets and rolls of various sizes and lengths.

Transfer Paper is a thin, tough paper, having one side covered with black, blue, vermilion, or other colored material. The colored face is laid next the paper on which the transfer is to be made, the copy placed on the back, and the lines fol lowed with a stylus, which produces the transfer.

Profile Papers (Fig. 1), in sheets, rolls, or made into

FIGURE 1.

books, are used in plotting profiles directly to scale. The distance apart of the horizontal line is usually from a fifth to a tenth of that of the verticals; this is because vertical distances are usually small as compared to horizontal, and in order to show small differences of elevation. It is called the exaggeration of the vertical scale.

Cross-Section Papers (Fig. 2), in sheets, rolls, or made

FIGURE 2.

into books, are also used in plotting directly to scale. The lines, both horizontal and vertical, are the same distance apart.

Both the above papers are ruled to parts of inches or metrically, and every fourth, fifth, or tenth line is made heavier for convenience in counting the spaces.

INKS.-Liquid indelible drawing inks are made in all colors and put up in bottles ready for use. Waterproofdrawing inks are useful when tinting is to be done. India ink is, how

ever, the best and most reliable. It comes in sticks (Fig. 4),

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FIGURE 4.

and is prepared for use by pouring a little water in a saucer or ink-dish and then rubbing one end of the stick in it until a jet black is produced.

It is the most suitable ink for brush work. For drawings to be photographed the ink should be dead black. For those on transparent paper, to be reproduced by the blue process, it is recommended by some to add a few drops of crimson lake. To erase ink lines, if still wet, first use blotting paper, then let the line dry; put the paper on a hard smooth surface and, with a needle point cr fine pointed sharp knife blade, carefully remove the line by lightly scratching it back and forth, disturbing the surface of the paper as little as possible; afterwards the rubber eraser is used over it and the surface of the paper rubbed smooth with bone, ivory, or the thumb nail.

DRAWING INSTRUMENTS.-In the selection and purchase of drawing instruments it is advisable to get only the best, ever at a greater cost, although one who intends to use them only occasionally and to work with them without particular reference to the excellence of execution might get along with a cheaper grade than one anxious to accustom himself to doing the best and most accurate work.

The instruments most necessary in plotting notes of a survey are thumb tacks (Fig. 5), pencils, pens, a straight-edged Т

FIGURE 5.

ruler, right-angled triangle, T-square, dividers, compasses, protractor, and a scale of equal parts. Besides these, there are