Pendulum Sextant
   Pendulum or gravity sextant 1.22050059595

Date manufactured: ca 1870

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.

Item History: Sextants are instruments used to measure angles. The most common form of sextant is made of brass and is used for navigating at sea. Sextants like the Hall Colby sextant are designed to be used over land for surveying and map making. Navigators at sea sight the horizon to establish a horizontal line of reference but on land, there is seldom a level horizon to use. Therefore, sextants for use on land need an artificial horizon. The Hall Colby sextant uses a free-swinging arc shaped pendulum to establish a vertical line of reference against which the angle is measured. The scale is divided 90–0-90 so that the sextant can measure angles up and down. Thus, it is technically incorrect to call the Hall Colby instrument a sextant, better called a “quadrant”.

Hall Colby of New York, New York patented their design, first in England in 1855 and an improved version in the US in 1870. The instrument shown here is dated 1870 and is of the improved design.

Supporting documentation:
Hall Colby Patent (2949696 bytes)

The Mariners' Museum
100 Museum Drive; Newport News, Virginia 23606

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Submission authored by:
Jeanne Willoz-Egnor
The Mariners' Museum
100 Museum Drive
Newport News, Virginia 23606
(757) 591-7764