TELEMETER, or RANGEFINDER. This is an instru- base is actually measured or found by means of a sub-base. ment used in modern warfare to determine the distance The range is obtained by table or calculating scale. The or range to an enemy's position, in order that correct Nolan rangefinder, which was the first telemeter used by elevations may be given to guns or rifles directed against the British artillery, was of this kind. (iii.) Where one it. Telemeters have been made on three distinct princi- base angle is a right angle, the other angles and base being pies, and classified as acoustic, optical, and trigonometrical variable. - The instrument used is generally double-reflectrespectively. ing of the sextant type, - the base being found as in (ii.).

Acoustic telemeters record the time which elapses between The most perfect example is the Watkin rangefinder, used seeing the flash or smoke and hearing the report of a gun, 14,,,i,.,.

lIONMIIIIIIIIII 6h2---: 4 49'P rifle, or shell, the range being given in yards as " the time ,..t .24.

in seconds x 364'6." The Boulenge telemeter is the best 0,,,,,,.

known of this class. It consists of a graduated glass tube filled with liquid and containing a small metal traki 4141,V11110114111412tibila..„ j yeller. At the flash the instrument is brought to a vertical 4.11111Niiiih._ position, and the traveller starts from zero ; at the detonation it is turned to a horizontal position and the traveller .... ,..zstops. The objections to the acoustic telemeter are that 4.6448 the rate of transmission of sound in air is affected by wind FIG. 2.-1Vatkin field rangefinder. - and other local conditions and that the instrument cannot by the British horse and field ar- 1,,,,, be used until firing has commenced. tillery. It (fig. 2) consists of an 4'a!9....-,.., •„„•••■•••••°1.11.

,....10"1" to any point by observing the size of some object of known two positions, and an index glass .. - _........

dimensions, as seen in a graduated telescope. Porro's set in a steel arm, which is worked by a movable collar telemeter, Elliott's telescope, and Nordenfelt's macro- on a graduated bar, and this again is moved by the turning meter illustrate the principle. The chief defect of the of a graduated cylinder. 0 (fig. 3) being the c.

system is that the objects most conveniently observed - object, the observer sets up a picket at A, and men and horses - vary considerably in size, so that the with the instrument at zero (the horizon glass assumption of a constant dimension may be productive being inclined 45° to the index glass) finds the of error. right angle at the point C. A sub-base AB of Trigonometrical telemeters shorten the ordinary methods 6 yards is then set off, and (with glasses set of surveying by adapting them to military purposes. They parallel and the sliding collar at 6) the obare of two kinds, - field rangefinders and rangefinders for server reflects B upon A by turning the cycoast batteries. linder, which is thus made to record the base (1) Field rangefinders exist in great variety, and differ AC in yards. This reading being set on the from one another both in the trigonometrical methods pur- graduated bar by moving the sliding collar, sued and in the mechanical peculiarities exhibited. The the observer proceeds to A, and from there following are the common solutions of what is technically reflects C upon 0, which causes the range to called "the range-finding triangle," - i.e., a triangle in be given in yards on the cylinder. In this which 0 (fig. 1) is the object the distance to operation the position of the sliding collar regulates the which is required, AOB an acute angle, and AB movement of the steel bar so that the number of turns of the base,-0 being visible both from A and B. the cylinder is always a true measure of the range OC, (i.) Where the base is a fixed length and the whatever the length of the base AC. (iv.) Where the angles are variable. - A fixed base is rarely angles are fixed and the base is a measure of the range. - adopted except when the base forms part of The base points are determined by the use of prisms or of the instrument, the angles being observed by A a mirrors reflecting the particular angles adopted. The base powerful telescopes. The range is usually read Fig. 1. is measured or found by a subsidiary triangle, and multi-in yards by the assistance of verniers, extreme perfection plied by a constant to give the range. The Weldon range-of mechanism being necessary. Many ingenious instru- finder, recently issued to the British infantry, is on this ments of the kind have been devised, but none have as principle. It consists of three prisms, and is generally yet proved satisfactory. With a fixed base the accuracy used as follows. 0 (fig. 4) being the object and D a con-diminishes as the range increases. (ii.) Where the base venient distant point, the observer makes with the first and the angles are variable. - The base angles are generally prism the right angle OAD. He then retires in the direcobserved by instruments of the theodolite type, and the tion DA till the second prism records the angle OBD = 88° 51' 15", when the range = 50 x AB. If it is inconvenient measuring line which can be stretched tight in a high to measure AB, the observer can retire from B in the line wind.

OB until the third prism records the C (2) Rangefinders for Coast Batteries. - Rangefinding is angle OCA =74° 53' 15", when the less adapted to the requirements of coast defence than range = 200 x BC. The prisms must " position finding," - a method which furnishes every gun be held in the plane of the objects with its proper training and elevation so that it can be and looked into at the same point. fired without sighting the target. Rangefinders are, howThis rangefinder is very simple and ever, sometimes employed. The most worthy of notice is portable, but is frequently inappli- the Watkin depression rangefinder used by the British cable on hilly or broken ground, and artillery in coast batteries. The instrument resembles in does not possess great accuracy. principle the Watkin field rangefinder, the height above The merits of different field range- the sea-level being a vertical base. The range is found by finders depend mainly upon the Fm. 4. - Weldon range- observing the angle of depression to the object. This is balance of advantages they offer finder. done by a powerful cross-wire telescope, which forms part of with respect to accuracy, suitability to variety of ground, the instrument. The fastest steamer can be continuously simplicity, portability, and durability, these conditions followed, and even the successive grazes of shot and shell being of a more or less conflicting character. The fol- can be observed. The instrument is levelled upon a tripod lowing are recognized principles : - (1) the naked eye stand. When necessary, it finds its exact height in feet cannot with certainty appreciate less than one minute dif- above the water-level in any state of tide by reference to ference of angle, therefore telescopic power is necessary a datum distance, and it records the range in yards auto-in proportion as the base is short compared with the matically on a graduated cylinder. An interesting con-range ; (2) telescopes of high power cannot be steadied trivance combining telemeter and gun-sight, applicable to by hand alone ; (3) the longer the base the more incon- guns in permanent emplacements over non-tidal waters, venient are any restrictions as to its length or direction ; has been tried in Italy. By means of a cam the hind-sight (4) it is a disadvantage to be compelled to traverse the of the gun is always maintained in the position necessary to line joining base points ; (5) the longest base which give the proper elevation in firing, so that it only remains it is convenient to measure by hand is that length of to make the sights cover the target. (A. -W. w.*) TELEPHONE