Patent GB-191213734-A (GB)
Abstract: 13,734. Barr, A., and Stroud, W. June 12. Modifying scale of readings.-Relates to means for indicating the correct tangent angle of elevation to be imparted, or for imparting the correct tangent angle of elevation, to a gun for varying altitudes of the target, and consists in employing in combination with a rangefinder or gun-sight a cam operated by the working handle of the range-finder or by the ranging gear of the gun, and adapted to produce relative movement between a specially constructed range or tangent elevation scale and a pointer or mark movable independently in accordance with the altitude of the target. The construction of the apparatus is based on the approximate law that the tangent elevation for any range at a given altitude may be derived from the corresponding tangent elevation at zero altitude by introducing a factor depending solely upon the given altitude. In the form shown in Fig. 1, the correct tangent elevation is indicated by means carried on a range finder. A drum 7 freely mounted on the range finder tube 1 is rotatable by gearing 3, 4 from the measuring mechanism of the instrument, and carries a helical camthread (or groove) 8 designed to convert the range finder scale into a scale of corresponding tangent elevations. This cam thread engages a projection 9 on a slide 10 carried in guides 5 on the tube 1, so that when the drum 7 is rotated, the slide, which carries a logarithmic scale 6 of tangent elevations, is displaced parallel to the axle of the tube 1. Freely mounted on the tube 1 is a drum 11 weighted at 13 so that it remains fixed relatively to the horizon as the tube 1 is rotated to aline the range-finder on the target. This drum therefore rotates relatively to. the tube 1 through an angle equal to the altitude of the target. On the drum 11 is a curve 14 adapted to act as a movable pointer to the scale 6, and so constructed that at any altitude it increases the tangent elevation reading on the scale 6 corresponding to zero altitude by the logarithm of the constant factor aforementioned for that altitude. The tangent elevation corrected for altitude is therefore indicated at the point on the scale 6 opposite the curve 14. To enable ranges to be read off from the scale 6 instead of tangent elevations, the tangent elevation numbers may be replaced by the corresponding horizontal range numbers, the scale graduations remaining the same. The curve actually employed on the drum 11 is modified to compensate for the departure from accuracy of the approximate law stated above. As the departure varies with the range, it is necessary to employ two or more curves on the drum 11, each designed for employment over a certain portion of the range scale 6. The curves may be differently coloured, the various portions of the scale to which they refer being coloured to correspond. The scale 6 may be fixed, and the drum 11 moved axially by the cam thread 8. In the form shown in Fig. 4, the apparatus is used in correct connexion with a gun-sight. The drum 7 carrying the cam thread 8 is adapted to be rotated by the ranging gear of the gun. When the sight has been laid on the target, the operation of elevating the gun with respect to the sight line, until the graduation on the scale 6 corresponding to the given range to the target rests opposite the curve 14 on the weighted altitude drum 11, imparts to the gun the correct tangent elevation for the altitude of the target.