Patent GB-320695-A (GB)
Title: Apparatus for use in combination or in conjunction with a sextant adapted for use as an artificial horizon and direction indicator
IPC: G01C1/10
Filed: 1928-07-24
Abstract: 320,695. Morrison, L. H. July 24, 1008. Sextants.-An artificial horizon and bearing indicator for a sextant comprises a reflecting plane horizontal surface, an arm carrying the sextant, a second arm fixed relatively to the first and carrying a horizontal line, the image of which in the reflecting surface can be observed in the sextant and forms an artificial horizon, and an azimuth ring with lubber line and pointer whereby the azimuth of the sextant-carrying arm can be read; the whole being supported in a gimbal or equivalent mounting. The reflecting surface is preferably formed by the plane polished surface of a gyroscope wheel having a vertical axis of rotation. In the form shown the gyro-wheel 1, Figs. 3 and 4, is provided with a bearing point resting on the cup shaped upper end of a member 5 which is screwed into a spindle 4 passing through a bearing 3 in the lower end of the casing 2 and adapted to be driven by an electric motor &c. 32. The bearing point for the gyroscope is screwed into a ring which is screwed into a seating in the gyro-wheel, so that the point can be readily replaced when worn. Surrounding the spindle 4 and connected thereto by a feather key 9 is a sleeve 8 forming a clutch member and having its upper surface so shaped that it can engage the outer surface of the wheel 1. Surrounding the sleeve 8 and held from rotation by a feather 12 engaging in a fixed bracket 13 is a sleeve 10 provided with a rack 14 engaged by a pinion 15 operated by a knob 21, Fig. 10, external to the casing 2. To spin the gyroscope the pinion 15 is turned to engage the clutch and the motor 32 is started. When the wheel 1 has sufficient velocity the clutch is disengaged and the wheel then continues to spin under its own momentum. The casing 2 is preferably provided with a spirit level so that the observer can tell when the axis of the gyroscope is vertical. The clutch 8 can be further raised into a position in which the wheel 1 is clamped against the cover 25, to facilitate transport &c. To reduce the resistance to the rotation of the wheel 1 the casing 2 is made airtight and can be evacuated through an aperture having a non-return valve 38. The upper plane surface of the wheel 1 is polished, but preferably a dulled or blackened area is left at one part of the circumference to enable an observer to estimate whether the velocity of the wheel is sufficient. The upper part of the casing 2 is provided with observation windows 26 and with a circular platform 27 resting in a ring 39 connected by pivots 41 to a gimbal ring 42 which is connected by pivots 43 to a frame 44 having at its lower end a leg 47 of square section passing into a support 48 secured to the deck of the ship &c. The leg 47 is provided with a number of holes through any one of which a pin 52 can be passed, to enable the height of the instrument to be adjusted. A pointer 37 on the platform 27 extends over a graduated azimuth ring 40 rotatably mounted in a groove in the ring 39. The ring 39 is marked with a lubber line and may also carry a diagram, Fig. 12, as described in Specification 167,075, [Class 97 (iii), Thermometers &c.], for convert. ing true courses to compass courses and vice versa. Secured to the casing 2 above the platform 27 are two arms 28, 30, the arm 28 serving as a support for the sextant and the arm 30 carrying at its outer end a graduated screen 31, Fig. 8, having a central zero line which constitutes the artificial horizon line. This screen has a plane diffusing outer surface and a concave graduated inner surface having its centre of curvature at the centre of the cardan suspension 41, 43. One edge of the screen is marked with angular graduations on each side of the zero line, the graduation numbers being inverted so as to be seen erect when viewed by reflection; the two halves of the screen may be differently coloured. The arrangement is such that a vertical plane containing the line of sight of the sextant passes through the centre of the cardan suspension and contains the graduated edge of the screen 31; this plane also contains the inner edges of the windows 26. To use the instrument to find the altitude and bearing of, for example, a star, the azimuth ring 40 is rotated until the reading indicating the ship's course is opposite the lubber line. The sextant is then directed towards the star and is adjusted until the image of the star coincides with that of the central line of the screen 31. The altitude is then obtained from the reading of the sextant, the line. of sight to the image of the screen being inclined at an angle, which is known from the construction of the instrument, to the horizontal plane. The pointer 37 indicates on the scale 40 the bearing of the star. In computing the altitude, corrections must be applied for such factors as precession of the gyroscope (evidenced by apparent oscillatory movement of the screen image), change of altitude of the star during the time occupied in taking a reading, &c. The manner of applying the corrections is fully described in the Specification. Precession of the gyroscope can be checked by providing one or more fine spring wires 53, Fig. 3, with which the wheel can be brought into contact by inclining the casing 2. In Fig. 10 the driving motor 32 is shown mounted on pivots 34 in a frame 33 secured to the casing 2, so that it can be reversed end for end to enable the gyroscope to be spun in the direction appropriate to the hemisphere in which the ship is situated. In a modified form of the apparatus, the mirror comprises a stationary disc maintained in the horizontal plane by the gyroscope.