Page:The New International Encyclopædia 1st ed. v. 07.djvu/97

* ENGINEERING. 81 out a right angle in the field is as follows: To lay out a right angle to the line AD at the point A, measure the distance Al) equal to four convenient units as feet or yards; then taking one end of a tape held at L) and the other at A, tighten the tape so that the distance XD shall be equal to 5 units and XA to 3. The angle NAD will be a right angle. By bisecting this angle, one of 45° can be obtained; by trisecting the latter, one of 15°, etc. ENGINEERING. To lay oil' an angle of 00° proceed as follows: At the point E on the line AEB, hold one end of a tape, and with any length, as EB, describe the are of a circle, as indicated in the cut; with the other end at B ami tin' same radius describe a second arc intersecting the first. From tiie point of intersection 1), draw the line DE; the angle DEB will then be 00°. By bisect ing this angle one of 30° can be obtained. By bisecting this, one of 15°, etc. Clearing of Ground. Various reasons fre- quently render necessary the clearing of ground in the field. In the ease of a position definitely taken up with a view to battle, it is eminently desirable that as much of the ground to the front as lies within rifle-range should be cleared so thoroughly that an enemy cannot take advantage of the cover afforded by the brush and trees in advancing on the position. Sometimes it is im- practicable to secure sufficient ground for the encampment of the troops without clearing. For cutting away brush the best tools are heavy knives, machetes, bill-hooks, and brush hooks. Fia. 4. Biumn-nooK. The removal of saplings and trees is usually done witn hatchets, axes, and saws where sufficient time and tools are available. Where the urgency is greater, trees may be felled by the explosion of charges of gun cotton or dynamite either in holes bored in the trees, or in charges suitably placed against the outside. Military Surveys, Topographical Recon- noissances, and Maps. As indicated above, where a regular survey of a tract of land is desiri'd for some military purpose, such as the construction of a fortification, or a map of a military reservation, the survey will be made in the same manner and under the same limitations as to time expended, accuracy required, and in- struments used, as a similar survey for any civil purpose. (See Surveying.) Where, however, a survey is desired for use in connection with the movements of troops, or with the planning of a battle, it is almost invariably found to be neces- sary to resort to hastier methods. These must depend upon the nature of the information de- sired and the time allowed. Much assistance can frequently be derived from existing maps of the territory to be traversed. These generally fall short of giving all the information desired, and when they are available must usually be sup- plemented by topographical reconnoissances. The latter can be made either on foot or on hor e back. In general, the information to be ob- tained is plotted as it is noted, with a view In having the map as nearly finished as may !"■ when the route to be mapped has been traversed. The essentials lor such a survey arc paper for the map, an instrument for measuring distances, and an instrument for determining elevations anil slopes. For many years such ma [is were, in the first place, plotted directly in note-1 1 on pieces of paper ruled in one direction and attached to a light drawing-board of such size that it could be readily carried by the recon- noitrer. The bearings of roads and objects were taken by a prismatic compass, or by a pocket- compass, and were plotted on the map with tie- aid of a small protractor. Distances are usually measured by counting the number of steps. Where a wheel of any kind accompanies the reconnoitrer, distances can be determined with reasonable accuracy by noting the number of rotations of the wheel by an odometer or other means. Differences in elevation of points are determined either by a hand level or by compu- tation from the slopes which are measured with a hand clinometer. Many little devices assist greatly in the rapid completion and easy under- standing of such a map. For example, if at the end of the day's work a red pencil be run over tne roads, a blue one over the rivers and other water surfaces, and a green over the forests, much will have been done to add to the clearness of the map. It was by these means that many of the to- pographical reconnoissances of the Civil War were made. The principal inconveniences of the method related to the size of the drawing-board and to the fact that provision must be made for carrying the various auxiliary instruments in convenient places about the person. In recent years some attempt has been made on the part of inventors to develop a sketching-ease which shall combine all the essential tools required for a topographical reconnoissance. One of the best so far developed is shown in the accompany- ing cut. It has been made possible in this case FlG. 5. BATBON SKETCHING-CASE. to use a much smaller drawing-board because the paper can be quickly rolled back and forth over it by means of the rollers on two opposite sides. The protractor can likewise be moved from side to side in a transverse direction. These two motions permit any point of the paper to be brought under the centre of the protractor The protractor can be revolved freely around its centre, thus permitting a line to he drawn in any direction through any point on the paper. The compass in one side of the board permits the board to be oriented, that is, placed in a position