340. In the example taken for illustration the point E, is the lowest point.

the level and take a reading upon a bench, which has

been determined to be 8.006 feet above some plane of reference, the sea level for instance. Suppose the reading to be 4.136, which is entered in the "+ Sight" column. The "Height of Instrument" will therefore be 12.142, as entered in the proper column. Next take readings upon E, D, and C3, reading C to thousandths as it will be used as a "turning point" (Art. 304). Remove the level to a new position and read C, again, 11.906, which gives a new "Height of Instrument" 22.441. From this position read as many stations as possible, and then use B, as a "turning point," and so on.

Plotting the Work.

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341. Draw, on a piece of paper, a straight line AE. From a scale of equal parts, set off distances AB, BC, &c., each to represent 100 feet. Erect perpendiculars to AE, at each of the points A, B, C, &c., and then set off the distances from A to 1, from 1 to 2, &c., each to represent 100 feet; and through the points 1, 2, 3, and 4, draw parallels to AE. These, by their intersections with the lines drawn through A, B, C, &c., will determine the position of the stakes A, 42, &c.; and write in red ink on the plot, the height above the plane of reference of each stake, taken from the column of total differences in the field-book. Let us suppose that the horizontal planes are to be taken at distances of 6 feet. We may find the points in which the contour lines intersect the sides of the rectangles, as in Example First. A very convenient scale for finding the points in which the contour lines cut the sides of the rectangles may be constructed thus: Upon any line as AB, Fig. 157, erect equidistant perpendiculars as at 0, 1, 2, &c. Parallel to AB draw lines, alternately heavy and light, as at 1, 2, 3, &c. Suppose we wish to find where the 12-foot plane cuts the side of a rectangle, CC, for example. The height of C is 10.5, and

of C it is 20.7, from which data we find the rise from C, to C

to be 10.2. The rise from C, to the 12-foot plane is 1.5.

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Now fasten a fine thread at A on the scale, and

stretch it to

Look along

cut 10.2 on the vertical scale, as AC in Fig. 157. the horizontal line 1.5, as dotted, and its intersection with AC will be at S, distant 16 ft. on the horizontal scale from A, or C. The tenths on the vertical and horizontal scales

may

be

estimated by the eye with sufficient accuracy. Engineer's Section Paper may be advantageously used for the above

purpose.

342. If only a rough plot is needed, the Surveyor may take the plot thus commenced, into the field, and by the eye trace the contour lines on the map. If we note where the lines of the rectangles cut fences, roads, streams, &c., we can, by joining the points, obtain a plot of the ground.

The coutour lines may be traced on the ground as follows: Set up the instrument and read a staff placed upon a bench, and determine the height of instrument above the datum plane. Sup

pose it to be 11.432. If we wish to mark out the contour line six feet above the datum plane, we set the target of the leveling rod at 11.432-6 5.432, and direct the rodman, by signals, up or down the hill, till the horizontal wire of the telescope coincides with the horizontal line of the vane. The foot of the staff is then 6 feet above the datum plane. Let a stake, marked 6, be driven here, and direct the rodman around the hill, until a second position shall be found, when the horizontal wire of the telescope will cut the vane, and drive there another stake, marked 6; and so on, until a sufficient number of stakes have been driven to determine the curve (6). Then, let the line of stakes, marked 6, be surveyed with the compass and chain, and plotted. Other contour lines may be found in a similar manner.

343. A practical application of this method is also called for when a surveyor is required to determine, in advance, the area of land which will be submerged by the construction of a dam. In this case the contour plane is fixed by the proposed height of the dam. Having set up at any point, a reading is taken upon the rod held upon a point at the proposed height of the dam, and the rodman is then directed to points along the shore of the proposed pond, or reservoir, which shall give the same reading. In this way the contour of the pond, when there is no overflow at the dam, can be determined. If the calculated depth of water upon the dam at the maximum overflow be added to the rod reading used above, the high-water contour may be also traced

out.

344. When the plane of reference is so chosen that points of the work fall on different sides of it, all the references on one side are called positive, and those on the other, negative. The curves having a negative reference are distinguished by placing the minus sign before the number; thus().

Shading and Delineation.

345. Fig. 159 represents a piece of ground sloping towards D, which is the lowest point; and through this point the plane of reference is supposed to pass. The following table indicates the heights of the several points above the plane of reference.