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Fig. 5.—MAP WITH 50-FOOT SQUARES, AND CONTOUR LINES.

fractions of a foot, and is furnished with a movable tar get, so painted that its center point may be plainly seen. The attendant raises and lowers the target, until it comes exactly in the line of sight; its height on the rod denotes the height of the instrument above the level of the ground at that stake, and, as the height of the instrument

Fig. 6.-LEVELLING INSTRUMENT.*

above the imaginary plane has been reached, by subtract ing one elevation from the other, the operator determines the height of the ground at that stake above the imaginary plane, which is called the "datum."

The next operation is to trace, on the plan, lines following the same level, wherever the land is of the proper height for its surface to meet them. For the purpose of illustrating this operation, lines at intervals of elevation of

The instrument from which this cut was taken, (as also Fig. 7,) was made by Messrs. Blunt & Nichols, Water-st., N. Y, 1867.

one foot are traced on the plan in Fig. 8. And these lines show, with sufficient accuracy for practical purposes, the elevation and rate of inclination of all parts

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of the field,-where it is level or nearly so, where its rise is rapid, and where slight. As the land rises one foot from the position of one line to the position of the line next above it, where the distance from one line to the next is great, the land is more nearly level, and when it is short the inclination is steeper. For instance, in the southwest corner of the plan, the land is nearly level to the 2-foot line; it rises slowly to the center of the field, and to the eastern side about one-fourth of the distance from the southern boundary, while an elevation coming down between these two valleys, and others skirting the west side of the former one and the southern side of the latter, are indicated by the greater nearness of the lines. The points at which the contour lines cross the section lines are found in the following manner: On the second line from the west side of the field we find the elevations of the 4th, 5th and 6th stakes from the southern boundary to be 1.9, 3.3, and 5.1. The contour lines, representing points of elevation of 2, 3, 4, and 5 feet above the datum line, will cross the 50-foot lines at their intersections, only where these intersections are marked in even feet. When they are marked with fractions of a foot, the lines must be made to cross at points between two intersections,―nearer to one or the other, according to their elevations,-thus between 1.9 Fig. 7.-LEVEL and 3.3, the 2-foot and 3-foot contour lines must cross.

LING ROD.

The total difference of elevation, between the

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two points is 3.3-1.9-1.4; } of the space must be given to the even foot between the lines, and the 2-foot line should be of the space above the point 1.9;-the 3-foot line will then come below the point 3.3. In the same manner, the line from 3.3 to 5.1 is divided into 18 parts, of which 10 go to the space between the 4. and 5. lines, 7 are between 3.3 and the 4-foot line, and 1 between the 5-foot line and 5.1.

With these maps, made from observations taken in the field, we are prepared to lay down, on paper, our system of drainage, and to mature a plan which shall do the necessary work with the least expenditure of labor and material. The more thoroughly this plan is considered, the more economical and effective will be the work. Having already obtained the needed information, and having it al., before us, we can determine exactly the location and size of each drain, and arrange, before hand, for a rapid and satisfactory execution of the work. The only thing that may interfere with the perfect application of the plan, is the presence of masses of underground rock, within the depth to which the drains are to be laid.* Where these are supposed to exist, soundings should be made, by driving a -inch pointed steel rod to the rock, or to a depth of five feet where the rock falls away. By this means, measuring the distance from the soundings to the ranges of the stakes, we can denote on the map the shape and depth of sunken rocks. The shaded spot on the east side of the map, (Fig. 8,) indicates a rock three feet from the surface, which will be assumed to have been explored by sounding.

In most cases, it will be sufficient to have contour lines taken only at intervals of two feet, and, owing to the smallness of the scale on which these maps are engraved, and to avoid complication in the finished plan, where so

*The slight deviations caused by carrying the drains around large stones, which are found in cutting the ditches, do not affect the general arrangement of the lines.

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