254,351. Grotendorst, W. F. Jan. 5, 1925. Plane-trigonometry calculators.-Apparatus for use in determining values for indirectly laying guns by observations from one or two observation posts comprises a table or support for a chart graduated in degrees and minutes, two or more pivotally mounted rulers, some of which are graduated to represent distances, a compass card mounted at the pivotal point of the rulers, and an indicating ruler shaped to represent the triangle formed by the observation posts and the gun position. As shown diagrammatically in Fig. 1, D represents the target, Z<1>, Z<2> two observation posts, and K the gun position. The problem solved by the apparatus consists in calculating the unknown quantities, vertical angle (t) and distance Z<2>D, from the known variable angles alpha, # and base Z<1> Z<2> (b), and from these calculated values the parallax angle (P) and the range DK can be determined. The distance Z<2>D is preferably obtained by means of the tables of angles and distances and a ruler as described in Specification 254,589. The table supporting the chart 15, Fig. 3, has a vertical pivot 16 on which are mounted the rulers 17, 18. The chart is graduated in degrees and minutes on either side of a fixed radial line graduated in distances, the ruler 17 is set to the calculated vertical angle (t) and the indicating ruler 20 representing the triangle Z<1>Z<2>K is placed with one end on the ruler 17 and the pointer 25 on the radial line, so that by positioning the other ruler 18 to coincide with the other end of the ruler 20 the range and parallax angle can be determined. The compass card 19 is set to correspond with that at the observation post Z<2>. The rule 18 carries an adjustable scale of distances for correcting the range for the various ballistic characteristics and corrections for barometer, temperature, wind, &c., and a graduated sector 23 with an adjustable pointer for setting lateral corrections. Corrections are subsequently made on the scales 22 and 23 for the longitudinal displacement of the target. When firing at small distances the observation post Z<1> is omitted and a range finder is used at Z<2> so that the angle # and distance Z<2>D can be determined directly. In this case the ruler 17, Fig. 5, is set to the angle # and a distance OB is marked along the ruler to represent the known distance Z<1>Z<2>. Lines parallel to the radial line are marked on the chart and the indicating ruler 20 is placed with one end coinciding with the line passing through B, the other parts being adjusted as above-described. The ruler 20 may be telescopic and the pointer 25 adjustable to suit variations in the disposition of the points Z<1>, Z<2>, and K. With two observation posts the simplified form of apparatus may be used with the addition of a third ruler which can be set to the vertical angle (t), the left-hand end of the indicating ruler 20 being then set to coincide with the intersection of the third ruler and the line through the point B. A graduated ring and plate at the gun position oriented with respect to the direction Z<1>Z<2> is used to set the angles #, P. Specifications 254,588 and 254,590 also are referred to.