at night, the two caps of the night-glasses should be opened. On
looking through the instrument, any lamp or other light will
appear like a fine, bright line, and the range can be taken in the
ordinary way.
This range-finder possesses the superlative advantage of the one-man instrument, and it is claimed for it that it can range on horizontal objects, such as the crest of a hill, which has no detail suitable for use with a mekometer, and that it can be adjusted on service with no greater difficulty than the setting of a watch.
3. For harbour defence, owing to the long range of naval guns, and the fast targets which war-vessels present, an accurate range-finder is of first importance. This is largely the case because “ranging” cannot be resorted to in the same manner as in the field, where the targets are comparatively motionless and the effective ranges are less. Successful artillery practice therefore depends, in a great measure, upon the range-finder. The instrument used in harbour forts is known as the depression range-finder. As its name suggests, it solves a triangle in the vertical plane, of which the base is the height of the instrument above sea-level. Its appearance resembles some forms of theodolite (fig. 6). A framework, capable of rotating in azimuth on a vertical
Fig. 6.—Depression Range-Finder.
ivot, is supported on a plate carried by levelling screws, L, L, L. To the framework are pivoted two arms DC and FE, at C and E respectively. The arm EF is supported at F by a vertical screw H ending in a drum, upon which, in a spiral scale, the ranges are graduated. Motion in altitude is thus given to the telescope. The arm CD is supported by a slider G. This slider is set by a rack and pinion to the height above sea-level (represented on a scale of feet on EF) at which the instrument may be used. A telescope AB is suitably fitted in jaws at the top of the frame. There are spirit levels at M and Q for adjusting purposes. The telescope is provided with cross wires which can be illuminated for night use. An azimuth circle X and pointer Y enable the direction of any vessel to be indicated, the range of which it is desired to know. The instrument rests on a base plate R, to which it is locked by the top-plate O. The observer directs the cross wires of the telescope upon the water-line of the objective, by means of the drum I and the azimuth handle P, the top of which just appears in the diagram. The reader watches the arrow on the drum and calls out the ranges as the figures arrive beneath it. 'The ranges are communicated to the officers at the guns by various devices, which differ according to local requirements.
Position-Finder.—The range-finding instrument known in the British service as the Position-Finder (invented by Colonel Watkin, C.B., R.A.) is practically a large depression rangefinder. It posesses, however, certain additional appliances which render it capable of automatically recording, upon an oriented chart, the position or course of a vessel. And further, by electrical means it automatically records to a distant battery the range and bearing of the desired objective. The position-finder can therefore, from a concealed and safe position, Coast Defence Instruments. automatically control the fire of a group of guns, whose detachments need not necessarily see the target engaged. As the observer follows the objective with the telescope of the instrument the range and bearing is simultaneously shown in the battery on convenient dials. The distance and direction thus communicated are the range and bearing from the guns, not as measured from the range-finder. The correction due to the displacement between gun and instrument is automatic. In localities where the height does not admit of using the depression system, an alternative arrangement is provided, known as the Horizontal Position-Finder. It is open to the objections common to two-man range-f1nders, and is only employed where necessity compels its use. Briefly, there are two observing stations at either end of a measured and electrically connected base. One is known as the transmitting and the other the receiving station; the latter contains the principal instrument, which usually is capable of independent use for medium and short ranges as a depression instrument.
It will be seen that the difference between the two systems is, that the first described solves the range triangle in the vertical, and the latter in the horizontal plane. There have been various methods proposed for using the position-finder. The best results are obtained by placing range and bearing dials on the gun-mounting in a position where they can be easily seen by the men elevating and training the gun. The gun is kept directed upon the objective and fired as quickly as it can be loaded. A position-finder can be used for firing mines in a mine field, and instruments are issued to the Royal Navy for this purpose.
In the United States of America the term “position finder” is applied to a range-finder which gives direction as well as distance. This is substantially correct, but custom, in the British service, confines the use of the expression as defined above.
4. Various appliances, not strictly range-finders, are sometimes used to assist in estimating distance. The following examples are not without interest:—
Acoustic telemeters, depending upon the velocity of sound, are obviously unsuited to the requirements of modern warfare. The names of Thouvenin, Rédier and Le Boulengé are connected with such instruments-that of the last-named is perhaps the most convenient. It consists of a graduated glass tube filled with liquid, of suitable density, and containing a small metal traveller. At the flash of discharge of a gun or rifle the instrument is brought to a vertical position, and the traveller starts from zero; at the detonation, it is turned to a horizontal position and the traveller stops at the point on the scale indicating the range.
On this principle is the rough method of ascertaining the distance, in yards, of a thunderstorm, viz. multiply the number of seconds elapsing between the perception of the lightning and that of the thunder by the number of days in the year.
Optical or perspective telemeters determine the distance to any point by observing the size of some ob'ect of known dimensions, as seen in a graduated telescope. Porro's telemeter, Elliott's telescope and Nordenfelt's macro meter illustrate the principle. Other Telemeters. The chief defect of the system is that the objects most conveniently observed—men and horses—vary considerably in size, so that the assumption of a constant dimension may be productive of error.
On the continent of Europe the perspective telemeter for military purposes has attracted more attention than in England. The French in their precise terminology call such an instrument “Stadia militaire,” a term which at once distinguishes it from a “télémetre,” and describes its nature. In rapid military sketching, in locating positions upon maps, &c., perspective telemeters find a use. The telescopes issued to field batteries and to coast forts in Franceare provided with a scale in the field of view. By comparing this scale with known heights, such as the average height of a man on foot, or the known height of funnels, masts, turrets, &c., of a war-vessel, distance can be estimated with fair accuracy.
The “jumelle Souchier,” which can be usedas an ordinary field glass, is constructed on the stadia principle. By its means ranges can be estimated within an accuracy of 10%. A stand or rest, however, is necessary for good results.
General Percin of the French army has shown, in an interesting pamphlet, that a piece of wood or card cut to a known fraction of the distance between the eye and the end of the thumb, when the arm is fully extended, can be used to estimate distances. Thus it is easy to find a penny in good condition of which the thickness is 1400th part of the arm-length in a man of average height. Provided with such a coin an observer finds its rim to exactly cover a distant man 6 ft. (or 2 yds. high). The range therefore is 400×2 = 800 yds. Similarly, if the man's height appeared to be but half the thickness of the coin the range would be 4×400 = 1600 yds. With a little practice the eye estimates the proportion between the object of
