Telemeter

An instrument for determining long distances. (See Range-finder .) One of the best-known telemeters is the invention of Capt. A. Gautier of the French army. It consists of a short tube containing two mirrors set at an angle of 45° with each other, one of which is fixed; the other admits of a slight rotation. A hole in the side of the tube allows the image of a secondary distant object on the prolongation of the base-line to be brought in line with that of the distant object whose distance is to be measured. An observation is next taken from the other end of the short base-line, and the image of the secondary object again brought in contact with that of the principal object by rotating a ring on the front of the tube. The extent of this rotation (as denoted by a scale), gives a factor which, multiplied by the base-line, gives the required distance. The Boulongè telemeter is an instrument devised for ascertaining the distance to a point by means of sound proceeding from the point to the place of observation. The one used for artillery consists of a gla tube about 6 inches in length, filled with a transparent liquid that does not freeze except with intense cold. In the liquid is a metallic disk, which moves freely from one end of the tube to the other. It is so adjusted that the motion will be uniform and comparatively slow. The tube is inclosed in a bra case, to which is attached a scale, after the fashion of a thermometer. This scale is marked for each hundred yards up to 4000. The divisions on the scale show the distance, in yards, through which sound will travel in air during the time required for the disk to descend over the space on the scale marked by the corresponding number of yards. The instrument must be held vertically, or as nearly so as po ible. To arrest the motion of the disk at any point, the instrument is quickly turned to a horizontal position. To use it for determining the time of flight of shells it is held in the right hand, back of the hand up, with the zero of the instrument to the left; a turn of the wrist to the right brings the instrument vertical, with the zero end uppermost; the disk then descends, and a turn of the wrist to the left arrests its motion. The observer, holding the instrument as described, watches for the flash of the shell, and upon seeing it instantly brings the instrument to a vertical position; upon hearing the report from the shell he instantly turns it back again. The position of the disk indicates the number of yards from the observer to where the shell exploded. To ascertain the distance to an enemy’s battery, the instrument is held and turned in the same manner. The observer watches for the flash of a gun; observing which, he turns the instrument, and when he hears the report turns it back and reads off the distance. Each hundred yards on the scale is subdivided into quarters. The telemeter invented by Capt. A. Gautier of the French army is an instrument for measuring, with a great degree of approximation, any difference, not exceeding three degrees, which may be exhibited in the bearing of a distant object by viewing it from different points of a base-line transverse to its general direction from the observer. The instrument, in its simplicity, accuracy, and portability, recommends itself in all cases where a knowledge of distances is desired at any moment and with the least po ible delay; such, for instance, as range-finding, river-cro ing, reconnoitring, and the like. A slight acquaintance with its use on such occasions enables the observer to estimate, with more than ordinary promptitude and precision, the distance which it might be all-important to obtain. The instrument resembles in shape and size one barrel of an ordinary reconnoitring or field-gla . The case in which it is carried is fashioned so as to answer as a handle for holding the instrument when making observations. Within the barrel of the instrument are placed two mirrors at an angle of 45° with each other; this angle can be varied within certain limits by means of a milled-headed screw acting on one of them. The mirrors are thus made to operate upon the principle of the sextant. A slot on one side of the barrel permits the rays of light from an object to fall upon one of the mirrors, from whence they are reflected upon the other mirror, and the image is seen through the eye-gla at the small end of the instrument. At the front or large end is fixed, in a ring surrounding the barrel, a prism, whose displacement modifies the direction of an object seen through it. At the rear of the instrument is a small eye-gla , by means of which the observer sees, over the mirrors and through the prism, the object which is before him, and by double reflection in the mirrors the object to the side of him. The American general Berdan has invented a large telemeter for garrison and sea-coast service which has been tested in Germany in 1875 and 1876 and found to be very exact in the determination of distances. He has also constructed a new model for field and mountain artillery which can be packed up and transported on horseback.