GB130930A - Improvements in or relating to Means or Apparatus suitable for use in Solving Problems in Nautical Astronomy, Navigation and Spherical Triangles. - Google Patents

Abstract

130,930. Thurburn, T. H. April 4, 1919. Spherical-trigonometry calculators. -Apparatus for use in solving problems in nautical astronomy, navigation, and spherical-trigonometry, and more especially for determining the circle of position or Sumner line of a ship at sea, comprises a frame carrying adjustable gnomonic charts adapted to represent the latitude corresponding to the assumed position of a ship and the declination corresponding to the subsolar or substellar point, and a pair of adjustable telescopic compass legs adapted to measure the angular distance between two points on the charts or to draw a circle of position on the latitude chart given the subsolar or substellar point and the true altitude of the body. The charts are mounted respectively on a fixed table and a movable table angularly adjustable with respect thereto and are placed at a distance, equal to the radius used in constructing the charts, from a " fixed point," representing the centre of a .great circle, and are mounted at right-angles to the line joining the " fixed point " and the " point of contact " of the charts. The charts are rotatably mounted on their tables so that their relative positions in azimuth can be correctly adjusted and a stationary index line and observation window is provided on each table. As shown in Figs. 1 and 2, the vertical frame a is provided with a horizontal stationary table c on which is mounted the latitude chart A, Fig. 12, a movable table d on which is mounted the declination chart B, Fig. 13, and a projecting spindle b carrying the universal bearing 3 of the compass legs 1, 2. The table d is carried by a frame e mounted to turn about the axis xx passing through the fixed point of the apparatus, and is adapted to slide over a quadrant a<1> having an arc f graduated in degrees and minutes and can be accurately adjusted by a rod e<1>. clamping-screw i, tangent screw k, and nut n. Frames r, r<1> are hinged to the tables and each has a window s of transparent material provided with an index line s'. The tables are supported by rods u, u<1> which can be disengaged to allow the tables to be turned to an inoperative position. The compass legs are jointed as shown in Fig. 3, and a coiled spring 9<1> tends to move the legs apart against the pull of a steel tape 10 passing round a drum 11 and secured thereto by a set-screw 12. The latitude charts A have a series of concentric circles D - - L representing circles of declination and graduated in hour-angle azimuths. The declination charts B are provided with similar circles D<1> - - L<1> representing degrees of latitude and graduated in hour-angle azimuths to represent the angle through which the latitude chart must be turned to suit the declination chart in use. Adjacent the hour-angle graduations are figures, preferably in red ink, representing the corresponding zenith distances. In order that the table d may be brought over the table c the radius of construction of the chart B is slightly less than that of the chart A. To admit of the circle of position being drawn when. the table d overlaps the latitude chart A, there is fixed to the frame e below the table d an auxiliary table d<1> provided with an arm d<2> slidable with respect to a clamping-screw and carrying a transparent window d<3> which can be adjusted to the substellar point of body marked on the chart B when the table d is turned to its inoperative position. To lay off or measure a. bearing, azimuth, or course, one of the compass legs may be provided with a carrier 24, Fig. 15, pivoted about the axis passing through the pin 16 or pencil 18 and the fixed point and having a needle 26, the prolongation of which will pass through the fixed point. An index 28 is provided which points to the zero mark of a protractor scale 27 when the needle 26 is in a line with the compass legs. The needle 26 is then moved to the same meridian as the pin 16 and the angle read off in degrees on the scale 27. Modifications are described in which a single gnomonic chart may be employed, or the charts may be mounted on the outside of a circle representing a great circle of the earth, and instead of the compass legs a trammel-bar may be substituted.