Accession No
1295
Brief Description
sextant, with stand, by Troughton, English, 1790 (c)
Origin
England; London
Maker
Troughton
Class
navigation
Earliest Date
1790
Latest Date
1790
Inscription Date
Material
metal (brass); glass; wood
Dimensions
box length 380mm; breadth 347mm; height 171mm
Special Collection
Provenance
Belonged to the Cavendish Laboratory(?)
Inscription
‘Troughton, London’
‘200’
Description Notes
Sextant, with stand, by Troughton, English, 1790 (c).
Brass pillar frame sextant. Index mirror, with 3 shades. Horizon glass, with 3 shades. Screw-on telescope sights with lateral adjustment by knurled screw. Reinforced brass index arm, with attached reading glass on arm and type-B vernier on brass, clamp and tangent screw. Scale divided on brass -5-140° by 1°, subdivided to 10’. Wooden handle in permanent brass mount and pierced for mounting on stand (securing nut missing). Three brass feet. Brass stand with counter-weights and three radial feet with two levelling screws (one missing). (Wooden mercury bottle with brass cap and artificial horizon, possibly not original.)
Condition: fair (one lens cracked).
References
Joshua Nall; ‘Instrument Breakage and Repair’; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2020: https://www.whipplemuseum.cam.ac.uk/explore-whipple-collections/astronomy-and-empire/instrument-breakage-and-repair
Events
Description
All sextants measure the angle between two distant objects. The telescope faces the clear portion of a small half-silvered glass, while a mirror is fixed to the index arm, its image reflected on the silvered portion of the half-silvered glass. The object is sighted through the telescope while adjusting the index arm until the direct and reflected images coincide. The index arm is clamped and the bearing of the object is then read off the scale of 60° graduated on the arc. By this double reflection, angles of up to 120 degrees may be measured. When measuring the sun’s altitude, the observer needs to look directly at the sun, which is harmful to the eyes. Tinted glass shades can be positioned between the telescope and the horizon mirror, reduce the sun’s power.
This sextant was intended for astronomy, and its frame can be tilted to any angle between vertical and horizontal, being counterbalanced by the two cylindrical weights on the back of the frame.
04/08/2008
Created by: Dr Anita McConnell on 04/08/2008
FM:39667
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