Accession No
1611
Brief Description
sextant, by Dollond, English, 1850 (c)
Origin
England; London
Maker
Dollond
Class
navigation
Earliest Date
1850
Latest Date
1850
Inscription Date
Material
wood (ebony, mahogany, other); metal (brass); glass; ivory
Dimensions
box length 336mm; breadth 380mm; height 125mm
Special Collection
Provenance
On loan from Trinity Hall, University of Cambridge from 16/03/1972.
Inscription
‘Dollond London’
Description Notes
Sextant, by Dollond, English, 1850 (c).
Ebony frame sextant, with brass fittings. Adjustable index mirror with four shades. Horizon glass adjustable to plane of frame by knurled screw, and lever adjustment of zero moved by second knurled screw, with 3 shades. Knurled screw for lateral movement of telescope mount (missing). Telescope missing. Reinforced brass index arm, with subsidiary arm for reading glass (missing), type-B ivory vernier, clamp and tangent screws. Ivory scale divided -2-139 by 1˚, subdivided to 20’. Ivory nameplate (unmarked). Wooden handle fixed in brass mount. Three brass feet. Keystone mahogany box.
References
Events
Description
All sextants measure the angle between two distant objects. The navigator usually needs to measure the height of the sun, moon or some star above the horizon. The telescope faces the clear portion of a small half-silvered glass (the horizon mirror), while a mirror is fixed to the rotatable index arm. The star or other object is sighted through the telescope while adjusting the index arm until the direct image coincides with the same image reflected on the index mirror and then on the silvered portion of the horizon glass. The index arm is clamped on the arc, and the bearing of the object is then read off on the graduated scale of 60°, with the aid of the vernier. By this double reflection, angles of up to 120° may be measured.
When measuring the sun’s altitude, in order to calculate their latitude, 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, to reduce the sun’s power.
04/08/2008
Created by: Dr Anita McConnell on 04/08/2008
FM:42767
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