Subtract the first of these foresights from the backsight C, and add the result to the total difference of level for C, and enter the sum in the column T. D. L.

Subtract each of the following foresights from the next preceding foresight, and enter the result, with its proper sign, in the column D., and add it to the next preceding difference of level, and enter the sum in the column T. D. L.

As a check, subtract the foresight of B, from the backsight C1; the difference will be the height of B above C, which add to the former check number, which is the difference of level of C and C1, and the sum will be the total difference of level of B, and C1. Compare the explanations of this article with the field notes of the following article.

368. Plotting.

Let the annexed diagram be a plot of the ground on which is written, with red ink, the height to tenths, taken from the field notes, of the surface, at each stake, above the plane of reference passing through C1.

To determine this point, observe that the rise from

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C1 to B1 is 7 feet. Then the distance on this line from C1 to the point where the height above C1 is 4 feet is found by the proportion,

7 4 100: x, .'. x= = 57.1.

This method assumes the ascent to be uniform between B, and C1; but this point can be tested and other points of the curve found as follows: Set up the instrument at S, and make the backsight to C1 10.378, the same as before; then depress the vane on the rod 4 feet that is, to the reading 6.378.

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Now let the rodman set up the rod at the point between C, and B, determined from the proportion, and let the surveyor observe whether the horizontal wire of the telescope ranges with the horizontal line of the vane; if not, let the rod be moved a little toward B, or

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C1 till they do range, and at the point thus determined let a stake marked 4 be driven by the axman.

An inspection of the plot will show that the curve. passes between B, and C, at a distance from C, found from the proportion,

9.8.1 : 100 x, ... x=1.

Let the rodman advance toward this point, pausing at one or two intermediate points, and at this point, whose positions are definitely determined and marked.

In a similar manner determine where the curve crosses C2 C3 and trace it to D2.

In like manner, trace the curves of intersection of the surface and planes, 8 feet, 12 feet, and 16 feet above the plane of reference, and let these curves be marked on the ground by stakes numbered 8, 12, and. 16, respectively.

The horizontal distance of each stake from two sides of a square can be measured and recorded. From this record the surveyor can draw the curves on the plot as exhibited above.

370. Conventional Signs.

The following conventional, though not altogether arbitrary signs, are used to indicate objects worthy

371. Finishing a Map.

The points of compass are indicated as is usual, the top of the map denoting the north, etc., etc.

The meridian, both true and magnetic, should be drawn, and the variation of the needle indicated.

The lettering should be executed with care, after printed models of various styles.

The border may be made by a heavy line, relieved by a light parallel.

The title, in ornamental letters, should occupy one corner of the map, with the name of the locality, the dates of the survey and drawing, and the names of the surveyor and draughtsman.

The scale of horizontal distances, for finding and comparing distances on the map, and the scale of construction, used in the smallest measurements required in projecting dimensions in the drawing, should be accurately drawn in some convenient position within the border. Parallels of latitudes and meridians, in extended surveys, should be drawn in their true position.

BAROMETRIC HEIGHTS.

372. Preliminary Remarks.

The barometer affords an approximative method for finding the difference of level of two stations.

To attain to as great a degree of accuracy as possible, it is important to employ two good barometers, one at the lower and the other at the upper station.

Before using the barometers, they should be carefully compared by frequent trials, and the variation ascertained, which is to be allowed for in the observations.