GB599955A - Levelling instrument - Google Patents

Abstract

599,955. Levels. COWLEY, K. Oct. 22, 1945, No. 27720. Convention date, Aug. 9, 1944. [Class 97 (ii)] In a levellinsg instrument of the gravitystabilized mirror type, two series of mirrors having respectively an odd and even number of mirrors 9, 10, 11 and 26, 27 are disposed at right angles to and on opposite sides of a central vertical plane common to both systems, extending through the eyepiece 6 and defined by upright strip 8. One of the systems contains the gravity-stabilized mirror 11 mounted on pendulum 15 supported on knife-edges whose pivotal axes are aligned with the reflecting surface of mirror 11 and are perpendicularly disposed to the central plane cutting this latter at P. The mirrors are so disposed as to give equal light paths in the two systems which produce images, e.g. of a sighting vane held perpendicularly to the central plane, inverted with respect to each other. These images appear in the two sections of the field of view as a continuous line when the centre of the vane is at the same level as the optical centre C of the instrument. This optical centre is in the central plane and is the image of P after reflection by the mirrors 10, 9 or 27, 26 in those orders. Damping means, e.g. an eddy-current brake 19, 21 is provided to damp the vibrations of the pendulum. Casing 1 is arranged for mounting on stand 32 and is engageable with and horizontally rotatable about post 36 whose axis passes through the fixed point C. Post 36 operates a spring-actuated lever 22 which, when unoperated, engages with pendulum 15 to retain this latter at rest, e.g. when the instrument is not in use.