Range-finder

An instrument for determining ranges. There are several different principles which may be used. The distance may be measured, 1st, by the visual angle subtended by objects of known height; 2d, by the velocity of sound; 3d, the instrument may furnish a base-line in itself and solve a triangle in which the base and two adjacent angles are given. The term is also applied to instruments used to solve a triangle, the base of which is obtained by outside means. Range-finders constructed on the visual angle principle have been known for many years. Boulanger’s instrument uses the 2d principle. It consists of a gla tube closed at both ends filled with a liquid in which a small umbrella-shaped piece of metal is submerged. The tube is held vertically in the hand, the metal slowly sinks to the bottom. When the flash of the enemy’s gun is seen, the tube is inverted and the metal moves towards the other end. When the sound is heard, the tube is brought to the horizontal. The distance through which the piece of metal has moved gives the range by means of a scale on the side of the tube. Berdan’s range-finder is an expensive instrument using the 3d principle. It is mounted on a wagon, and intended to accompany either foot-troops or artillery. It has found great favor in Germany. Nolan’s range-finder consists of an instrument for automatically solving triangles. A similar thing was devised about 1870 by two American officers, Maj. Morgan of the 4th Artillery, and Capt. Lorain of the 3d Artillery. The most ingenious, complete range-finder has been proposed by Lieut. Gordon of the 4th Artillery. He uses two fixed angles and a variable base-line supplied by the instrument itself. The principal parts of Nolan’s range-finder are: Two instruments for measuring angles, one tape-line, and one reckoning cylinder. Each of the two instruments consist of two telescopes, which lie cro wise one above the other under an angle of about 90°; the smaller of the two has a long arm, with a vernier at one end; to the other a sector is fastened, which is divided up into degrees. By means of a screw, an angle of about 20° can be described by the upper or smaller telescope. The reckoning cylinder consists of a solid body and two rotating rings. The lower ring and the lower edge of the body are divided into 100 equal parts. On the upper ring are the logarithms of the figures, and on the upper edge of the body are the logarithms of the signs, from 6″ up to 2° 15′. To find the range, the instruments on their tripods are arranged at the end of the a umed base-line, which is perpendicular to the range; or the instruments may be attached to the right and left guns of a battery. The long telescopes are turned toward the object whose distance is to be found; the smaller ones upon each other, and the cro -threads of each made to cover the cro -lines on the leather disk through which each small telescope points. The coincidence obtained by directing the longer telescope on the object, the two angles at the base are determined; the base-line being measured, one side and two angles of the triangle are obtained. With this data recourse is then had to the reckoning cylinder. The arrow marked “band” is set on the figure that corresponds with the distance between the instruments or base-line,—say 34 yards; then set the arrow on the lower ring on the figure corresponding with the angle found through the instrument,—say 18°; then find the figure for the number of degrees of the other angle,—say 42° on the lower ring. Just above that is the figure 60 on the other division of the lower ring; coinciding with this on the lower edge of the upper ring is the distance, 1320 yards. The bases used are from 30 to 40 yards for a range of 2000 yards and over.