FIELD MEASUREMENTS. The rapid approxi mate measurement of distances is frequently necessary in field operations. For short dis tances resort is had to the use of the rule and tape. For longer distances, where both extreme points are accessible, measurement is ordinarily made by pacing, running over the line with a wheel, or by estimation with the eve. With practice many officers become extremely expert in the determination of distances by the eye. Where the distance to seine inaccessible point, as, for example, a tree on the opposite bank of a river, is required more accurately than would probably he given by simple estimation, a method frequently used is the following: To find the distance from B to 0, being on the B side of the river, lay off a line at right angles to OB, and stake some point on this line as C. At an intermediate point in the line BC, as D lay off a line at right angles to BC. Nark the point E where the line of sight from C to 0 crosses DE. Then, by similar triangles, having measured DE, DC, and CB, we will have BO : BC :: DE : DC. There fore, ItO ___ BC X DE ÷ DC. By a similar hut slightly more com plicated process, the height of a distant in accessible point can be determined. Other sim
ple and practical nodicoils for measuring slim fiat- distances are described in the ing manuals. One of the simplest ways of laying out a right angle in the field is as follows: To lay out a right angle to the line All at the point A, measure the distance AD equal to four convenient units as feet or yards; then taking one end of a tape held at D and the other at A, tighten the tape so that the distance Xl) shall he equal to 5 units and XA to 3. The angle XAD will be a right angle. By bisecting this angle, one of 45° can be obtained; by trisecting the latter, one of 15°, etc. 'to lay off an angle of 60° proceed as follows: At the point E on the line AE13, hold one end of a tape, and with any length, as EB, describe the arc of a circle', as indicated in the cut; with the other end at li and the same radius describe a second arc intersecting the first. From the point of intersection I), draw the line DE; the angle DEB will then be 60°. illy bisect ing this angle one of 30° can be obtained. By bisecting this, one of 15°, etc.