RANGE-FINDER. An instrument used in naval and military operation, to determine the horizontal distance of the object to be hit from the gun which is firing. They are of three prin cipal types, those furnishing their own bases for measurement, those depending upon a known di mension of the object aimed at, and those utiliz ing a time observation. Those of the first-named type have the most general application and are therefore the most de,irable, but satisfactory in struments are either very cumbersome or delicate, or are liable to give ranges considerably in error. The instruments of the second type may he made fairly accurate, but have a narrow application. Those of the third type are generally inaccurate and are not of much use since the introduction of smokeless powders and rapid-tiring guns.
The simplest method of obtaining range by utilizing a base at the point of observation is called Buckner's method and was devi-ed by Lieut. Buckner, of the United States Navy. lu the figure, AB is the mast of the ship, which is endeavoring to obtain the range.
A i, the water line, B the position of the ob server. and D the position of the object observed; AB is therefore the height of the observer above the water; AEDC is the surface of the water from A to C (in the visible horizon of the point B) ; and AD is the desired range. By means of suitable tables the range All corresponding to any height (AB) and angle (DBC) can be in stantly ascertained. It therefore is only neces sary to measure the angle DBC by means of a sextant or similar instrument. This method has the disadvantage that it removes the observer from the vicinity of the guns, requiring transmis sion of the range when it is ascertained; this takes time. and with rapidly moving ships is almost certain to cause error. Furthermore, if the enemy has more than one ship the confusion of ranges may be so serious as to render the ranges obtained nearly valueless. The form of auto-base range-finder largely used in fortifica tions has two observers at a short distance apart (this distance furnishing the base). Thus A and B are the observing stations, AB the known base. and AC or BC the range. The angles CAD and CBA and the distance 13A being known, the dis tances CA and CB are quickly determined from tables similar to those in Bowditch's Practical Yarigator for ascertaining the "Di,tance of an Object by Two Bearings." Most of the recent range-finders are constructed On the same gen eral principle. but are arranged for operation
by one person. The base is therefore much short ened, and this is -ought to he compensated by the magnification of the angle at C so that it will appear equal to a much huger one before the unaided eye. The most prominent range-finders of the auto-base type are the Barr and Stroud, Zeiss, Davis. Lewis, and Fiske ;turret type). Of these the first two have had widespread use and are considered fairly satisfactory and the others have given good results in experiments.
The commonest means of obtaining the distance of an object whose height is known is to measure the angle it subtends by means of a sextant. Thus let BC be the object whose height BC is known. Then by measuring the angle at A the distance AC may be at once picked out of a table of right triangles. For convenience, a special form of sextant, called a stadiineter, has been invented by Commander Fiske of the United States Navy. The reading of the instrument is in yards instead of in angular measure, so that the use of the tables is avoided.
The third type of range-finder is merely a time measurer which gives the interval between the ob served flash of the enemy's gun and the hearing of the report. As the velocity of sound in air is known, the distance may be readily determined. For convenience, the reading may be given in yards instead of seconds. The Watkins range finder is an instrument of this type and consists of a small glass tube containing a liquid and a moving index. When the Hash of the gun is seen the instrument, previously held upside down, is inverted and the index falls slowly through the liquid; when the sound of the gun is heard the instrument is turned to a horizontal position. The side of the tube is graduated and the gradua tion mark at which the index is seen shows the distance. Aside from errors of range due to faulty graduation and irregularities of the ve locity of sound in air, accurate determination of range by an instrument of this kind is practically impossible because of the rapidity of fire of mod ern guns, whereby the distinguishing of the sound of a particular gun is rendered very difficult.
For further information, see the articles on BALLISTICS and on GUNNERY; Proceedings of the United States Yaral Institute (particularly the "Professional Notes" in numbers 97 to 102) ; An nual of thc Office of Xaral Intelligence, United States Navy (particularly the volume for 1900).