Range-Finders

Monostatic Height-finders

are actually range-finders with the addition of computing gear, based on some form of the equa tion, height =range X sin. angle of sight (or elevation). The Barr and Stroud is a coincidence range-finder with a beautiful loga rithmic toothed cam gear. The French Lavallois is stereoscopic with a triangular link gear. Both these gears are so arranged that adjustment is only lost when the height alters, the necessary movement of the translational prism for range being effected auto matically by that part of the computing gear which is controlled by the elevation movement of the main tube. This principle is essential for all height-finders.

In an optical solution, Rottenburg and Willans (Br. Pat. 125164, 1916) split the Barr and Stroud prism into two halves, ro tating them in opposite directions through the angle of sight. The translational movement was then a measure of height, not range.

Long Base Height-finders.

These depend on the Bennett Pleydell or "roof" principle. In fig. i 1, T is an aerial object.

LAar and RAar are two planes (Bennett planes) capable of being rocked about horizontal axes Ll and Rr (Bennett axes) at the ends of a measured base LR, at right angles to it and parallel to each other. Whatever the po sition of T, the planes can be rocked about their axes so that their intersection Aa (the ridge of the "roof") passes through it. If LAR is a vertical plane cut ting the roof, then the perpendicular AP is the height of the ridge and therefore of T, wherever T may be along the ridge. If the base LR is b, then the height h= . The angles cotL cotR ART and ALT are known as "sweep" angles. In practice the instruments at L and R have telescopes mounted on universal joints, of which the main (Bennett) axes are horizontal and set at right angles to LR. Movement about these registers the Ben nett angles, while movement about the sweep axes allows the telescopes to be swept along the ridge without affecting the Bennett angles.

The formula can be solved in various ways to give the height. The latest British instruments are a copy of the French Puteaux, Modele 1917. A semicircular plate, covered with curves of Ben nett angles is attached to the telescope so as to swing with it about the Bennett axis. A vertically sliding pointer, engraved with a scale of heights, is carried on the body of the instrument and can be moved up or down to touch any curve on the plate. When L and R have laid their telescopes on the target, L tele phones his Bennett angle to R, who then moves the pointer to that curve and reads the height opposite the indicator of the height scale. Further if L telephones his angles continuously, R will find that his disc brings the corresponding curves to the height pointer as L calls them through ; if not, the height has altered. Cf. constant height constant coincidence of monostatic type.

The disadvantages of this system are similar to those of type (b) range-finders (q.v.). Hence, though this type has its uses, the monostatic is more generally relied on ; as it is quicker in opera tion, speed being essential, and there is no confusion of targets. BIBLIOGRAPHY.-F. J. Cheshire, "Rangefinder, Short-base," Dic tionary of Applied Physics, Vol. IV.; Alex. Gleichen, Theory of Modern Optical Instruments, Eng. trans.: by H. H. Emsley B.Sc. and W.

Swaine B.Sc. 1st Edition (H.M. Stationery Office, 1918) . Publica tions of Messrs. Barr and Stroud, Zeiss, Goerz, Hahn, Soc. Anon.

d'Optique et d'Instruments ellaute Precision; British and Foreign Pat. Specifications Military handbooks of Range-finders (H.M.

Stationery Office). (K. F. D.)