October i8, 1900] 



NATURE 



607 



RANGE-FINDERS} 

 ^AVAL and military authorities are agreed that for accurate 

 shooting almost everything depends upon the range being 

 known with sufficient exactitude. It is not surprising, therefore, 

 that an immense amount of attention should have been devoted 

 to methods of rapidly determining the ranges of distant objects, 

 whether stationary or in motion. These methods may be 

 classified as follows : — 



(1) J/cr/;a«/t-a/.— In this method a trial shot is fired from a 

 gun, and so far as is practicable observation is made as to 

 whether the shot strikes the ground (or the sea) on the near or 

 the far side of the target. The results of the observation are 

 used to correct the next trial shot, and so on. The method is 

 clumsy and (at all events on board ship) costly ; it is inapplic- 

 able in a naval engagement where the ranges frequently alter 

 with great rapidity, and is, of course, totally inapplicable for 

 purposes of navigation. On the other hand, it involves the use 

 of no instrument beyond those ordinarily used in warfare. 



(2) Flash and Sound Method.— W^x^ the time-interval between 

 seeing the flash and hearing the report from one of the enemy's 

 guns is measured by a suitable chronograph or, by starting to 

 count at the rate of eleven in three seconds immediately on 

 seemg the flash ; the number arrived at on hearing the report 

 will roughly give the number of hundreds of yards in the range. 

 This method is, of course, inapplicable amid the din of a 

 general engagement, and besides it permits the enemy to have 

 the first shot. 



(3) Optical Range-finders.— \w the determination of a distance 

 by an optical range-finder, length of base and titne available are 

 very important factors. Given plenty of base-length and plenty 

 of time, a theodolite, such as is used in surveying, satisfies all 

 the conditions except that of portability. However, time is 

 usually so important a factor for military operations that the 

 theodolite is completely excluded even for field service. If the 

 time is restricted, but the extent of base unrestricted, recourse 

 may be had to long-base, two-observer instruments, such as 

 those now in use by infantry and field artillery. When both base 

 and time are restricted, as they are on board ship— and moreover 

 the observer's station is itself in motion— the problem becomes 

 at once much more difficult and much more interesting. We 

 need not then be surprised that while military range-finders are 

 usually simple and cheap, naval range-finders are comparatively 

 costly and complicated. 



The operation of all optical range-finders depends upon the 

 measurement, by some device or other, of the angle subtended by 

 a known base-length at the distance to be determined, the 

 base being almost invariably " broadside-on " to the 

 point. Where the known base is at the target and the 

 point is at the observer's station, the operation of deter- 

 mining the distance consists simply in measuring the angle 

 subtended by the base at the point. Instruments of this 

 class measure, say, the angle subtended by the height of a man 

 (assumed to be of mean stature) at the observer's station, and 

 require for their successful employment the co-operation, willing 

 or otherwise, of the enemy, and are therefore seldom applicable 

 under the conditions of modern warfare. Men who have returned 

 from active service in South Africa have stated that sometimes 

 they never saw a Boer in an engagement. Where the known 

 base IS at the observer's station, which is an essential charac- 

 tensticof all range-finders of general application (the instruments 

 previously described being in reality mere angle-measurers), the 

 operation of determining the range consists almost invariably in 

 swingmg the base round until it is '* broadside-on " to the target, 

 and then measuring the angle subtended by the base at the 

 target. The measurement of the angle at the target is very 

 commonly effected by making one of the base-angles a right 

 angle, and observing how much the other is "off'" the right 

 angle (mekometer, &c.), or else by observing how much the sum 

 of the base-angles is less than two right angles (naval ranee- 

 finder, &c. ). 



Since a mere enumeration of the names of inventors of range- 

 finders would occupy the whole of the available time for a lecture, 

 It will be necessary to restrict the selection of examples to those 

 instruments which are, or promise to be, in actual operation 

 for warlike purposes. Attention then will be confined to the 

 following instruments : — 



(i) The Watkin mekometer, used by our troops in South 

 Africa. 



NO. r6i6, VOL. 62] 



(2) The Watkin depression range-finder, used at certain 

 stations for coast defence. 



(3) The Zeiss tele-stereoscope. 



(4) The Barr and Stroud range-finder, used in the British and 

 other navies, in coast fortifications, and to some extent in the 

 field. 



Restrictions of time unfortunately render it impossible to 

 describe the Watkin artillery range-finder, as well as the 

 Fiske instrument which is used to some extent by the U.S. 

 Navy. 



(I) Mekometer.— 1\):\% instrument consists of two parts con- 

 nected by a cord 25 or 50 yards long, which is kept tightly 

 stretched by two observers, each of whom supports one of the 

 parts. These two observers are designated respectively the right- 

 angle man and the range-taker. The former carries a small 

 optical square or instrument in which two mirrors are fixed at 

 45" so as to set out a right-angle. 



The part carried by the range-taker consists practically of a 

 box-sextant in which one of the mirrors is adjustable by means 

 of a graduated micrometer-screw, on which the ranges are 

 marked for the base specified, the infinity mark corresponding 

 to the case when the mirrors are exactly at 45°. Each of the 

 parts of the instrument carries a prominent mark in a suitable 

 position, and the operation of taking a range on a fixed object 

 is as follows :— The two observers supporting the instruments 

 connected by the taut cord set themselves in such positions that 

 the cord is nearly "broadside-on" to the target; the range- 

 taker now stands still, while the right-angle man— by moving 

 forwards or backwards, with the cord always taut— adjusts his 

 position accurately, so that the image of the mark on the range- 

 taker's instrument, seen through his optical square, coincides 

 with the target viewed directly. He now shouts " On," when 

 the range-taker adjusts his micrometer-screw so as to bring the 

 mark on the other instrument into coincidence with the target, 

 and then the reading on the graduated scale gives the range. 



For objects in motion the range-taker remains steady, while 

 the right-angle man, as far as possible, continuously shifts his 

 position so as to keep the mark on the range-taker's instrument 

 and the target in coincidence. In cases of rapid motion at right 

 angles to the line of sight, difficulties are experienced in keeping 

 continously " on," and in such cases the right-angle man shouts 

 "On" whenever the mark passes the target, and the range- 

 taker then seizes the opportunity of taking the range. 



There are several serious objections to two-observer instru- 

 ments. In the first place, they cannot be used for the measure- 

 ment of the distances of sky-lines, trenches, hedges, &c., of a 

 more or less horizontal character, with no prominent vertical 

 features about them. Secondly, it not infrequently happens 

 that the two observers are working on different objects altogether 

 or on different portions of the same object, in which case an 

 entirely fictitious range is obtained. Most important of all, 

 they necessarily expose the range-takers. It has been stated 

 that several casualties have occurred in South Africa among men 

 engaged in range-finding. Under such conditions as those per- 

 taining in South Africa, it is obviously most risky to expose men 

 in the open. Single-observer instruments are free from all 

 these defects. 



(2) Depression Range-finder. — li we have a vertical base, and 

 if the target is constrained to move in a horizontal plane, we 

 may dispense with the right-angle man altogether. The only 

 case where this plan can be adopted in practice is when we wish 

 to determine the range of a ship from a hill near the coast. 

 The base-line is now the vertical height of the observing station 

 above sea-level, and the range is obtained very simply by 

 observing the angular depression of the water-line of the ship. 

 Allowance has to be made for the effect which the tides produce 

 in varying the equivalent height of the base. Unfortunately, 

 the water-line of a ship — especially if the waves are at all high 

 — forms an exceeding bad object upon which to observe, and 

 this uncertainty can only be compensated by having a very long 

 base— /.<•. by having the range-finder at a very considerable 

 altitude, say 150 feet or so. We therefore cannot use such an 

 instrument on a flat coast, nor can we u«e it for determining 

 distances at night, unless we can sufficiently well illuminate the 

 water-line of a ship by means of a search-light. 



(3) Zeiss Range-finder.— Thh is the first of the two single- 

 observer instruments which it is proposed to describe in the pre- 

 scribed limitsof time. The credit of the idea underlying the con- 

 struction of the instrument is due to the late Herr Grousillier of the 

 German Army, but as the firm of Zeiss, of Jena, have spent many 



