The object glasses, halving and coincidence adjusters, astig matisers, eyepiece prisms and translational prism are carried in an "inner frame" "geometrically" supported within the main tube, which carries the eyepieces, end reflectors and, when used, rota tional prisms. The main tube may be carried on a mounting de signed to facilitate the use of the range-finder.
In the field small range-finders up to one metre base length are in use. They are handled by one man and are carried either by the man or on a horse or in a vehicle. In the case of the instruments used in ships and forts, which are much larger and are mounted on pedestals or in turrets, although one man is sufficient to "take the range," he is assisted by others in direct The triangle RTL has been solved electrically by employing the logarithmic sine formula (e.g., Pollak, J. E., Br. Pat. 218548). The azimuth instruments actuate rheostats to give the apex angle. Other rheostats then solve the logarithmic sine formula to give the range LT or RT. The plotting board system in various forms is in universal use.
In fig. 8 TT' are targets on the sea GT'T. Rgt represents the range-finder on a cliff gG. The bar gt't is levelled and represents the sea. The height screw Rg represents the height RG and can also allow for tide, while the arm Rt is pivoted at R and carries a ing the instrument and reading the scale. In battleships and cruisers a range-finder is usually carried in each gun turret, where it is subject to severe shocks.
descriptions of the various details see publications men tioned in the bibliography. (X.) (b) THE LONG HORIZONTAL BASE SYSTEM While details of the long horizontal base range-finding system are usually secret, the principle may be indicated. Fig. 7 shows a simple plotting table or plotter, covered with a chart. R and L represent two azimuth observing instruments at dis tant posts. The observers lay their telescopes on the target and telephone the angles to the plotter. Arms RT and LT, pro vided with suitable arcs r and 1, are set to these angles. Then their intersection T is the position of the target. If G is a gun position, a graduated arm GT with arc g will give the range and bearing of the target from the gun G. If the target moves,
T will trace its course on the chart, the speed can be ascertained, telescope which can be directed at the water-line of a target by moving a slider along gt. The triangles RGT'T and Rgt't are then similar and so the bar gt't may be graduated in ranges. Curvature of the earth and atmospheric refraction, indicated by dotted lines, may be compensated for by suitably curving the telescope arm Rt.
The pivot R may be displaced laterally from g to represent the distance of the range-finder from the gun. The triangle RGT of fig. 8 (q.v.) is then reproduced, and the range indicated will be that from the gun. If a chart is placed under the bar gt't, the position of T can be plotted and the instrument becomes a de pression position finder. The useful limits of depression instru ments are 4,000 to 5,000 yards for every ioo ft. of height above the sea, the accuracy of the "lay" being io-15 seconds of arc.
The type (b) and (c) instruments used in the British service, together with their electrical operating devices, and certain field range-finders now replaced by type (a) were invented by Col.
and data can be sent to the gun continuously. Thus the gun can be fired at a target which may be invisible from the gun position. Other observing stations such as R' and L' may be used alter natively to either R or L, if either or both of these cannot see the target or if the apex angle RTL becomes less than about ion. Thus the system is very flexible. Disadvantages are the necessary communications and the difficulty of ensuring that both observers are on the same target if several targets are close together; but great accuracy is possible. A simplification is to mount a tele scope on one of the arms and use the plotter as an azimuth observing instrument.
S. S. Watkin between 1886 and 1899. This was a stupendous achievement at a time when no good range-finder existed.
The factor which is liable to least alteration in the flight of an aeroplane is its height. Anti-aircraft guns therefore base their calculations on height and angle of sight (elevation) instead of range, and so require height-finders. These are of the types (a) and (b), q.v.