Description: Pendulum or gravity sextant with artificial horizon by unknown maker. The instrument was patented by Hall COLBY on February 22, 1870. The instrument is graduated from 0°-100° in 30’. The vernier is graduated from 0’-30’ 1/10’. There are two shades, one red and one green, from which it could be concluded that the instrument was only meant for use with stars. See British Patent 1736, 1855 Altimeter or Self-Adjusting Quadrant US Patent 100,119 of 1870 Improvement in Altitude Instrument. An instrument for determining at the same time the altitude and zenith distance of celestial or terrestrial objects. For additional information see Peter Ifland's "Taking the Stars" pages 107-108.
This instrument works without mirrors or lenses. The observer directs the instrument at the celestial body, by looking through two sights, one of which has two threads and two lightly colored glasses. While moving the frame towards the sky, a weighted ‘alidade’, through gravity, remains vertical. When the celestial body is sighted, the observer releases a spring by which the ‘alidade’ becomes fixed to the arc. The altitude and/or zenith distance can then be read from the arc.