Accession No
1472
Brief Description
12-inch Altazimuth theodolite, by Troughton & Simms, 1839
Origin
London; England
Maker
Troughton & Simms
Class
astronomy; surveying
Earliest Date
1839
Latest Date
1839
Inscription Date
1839
Material
metal (oxidised brass, brass, silver); glass; ivory
Dimensions
height 640mm; length 525mm; breadth 310mm level length 248mm; height 91mm; depth 29mm
Special Collection
Provenance
Transferred from the Department of Geography, University of Cambridge, in 05/1969.
Inscription
‘Troughton & Simms
LONDON. 1839.’()
Description Notes
Altazimuth theodolite, by Troughton & Simms, 1839.
Oxidised brass. Telescope with achromatic object glass in screw-fit brass cell, push-fit shade, rack and pinion focus moved by knurled screw, screw-fit eyepiece. Central brass reflecting disc moved by screw beneath barrel reflects light entering axis to cross hairs (missing). Horizontal axis carries telescope, vertical circle and counterweight, all reversible in bearings. Vertical circle divided on silver 0 - 90˚ four times, subdivided to 1˚ and to 10´; also numbered 0 - 360˚; 2 type-B verniers, 2 reading microscopes. Horizontal level surmounted by ivory scale divided 0 - 40 both sides on either side of centre to 1/20 in. Clamp, tangent and zero adjustment screws for vertical circle. Y bearings carried by 2 conical pillars on horizontal plate, the whole moving over horizontal circle divided on silver 0 - 360˚, subdivided to 1˚ and to 10´; 3 type-B verniers and 3 reading microscopes; clamp and tangent screws, 2 bubble levels. Flat tripod base with 3 levelling screws.
Striding level surmounted by ivory scale divided 0 - 70 both sides on either side of centre, subdivided to 1/30 in.
Condition
References
Events
Description
The theodolite allows the surveyor to measure horizontal or vertical angles. For horizontal angles, point A is sighted through the telescope and its bearing noted from the graduated circumference; point B is then sighted, and its bearing noted. The difference between the two measurements is the angle between them.
For measuring vertical angles, a horizontal point A is taken with the bubble-level on the telescope; with the telescope tilted to point B, the angle of elevation is read from the graduated arch supporting the telescope.
In this instrument, the telescope is raised on pillars, enabling it to sight below the horizon and close to the meridian. This extends its use to astronomy and mine surveying. The plate carrying the pillars and telescope rotates on a lower plate, its graduated silvered rim provides the horizontal bearing. Precise measurements are achieved by the provision of verniers and reading microscopes on both vertical and horizontal circles. The oxidized finish to the brass prevents sunlight from reflecting in the suveyor’s eyes.
The instrument was made by Troughton & Simms of London in 1839.
History of the Theodolite
Gemma Frisius proposed the idea behind the theodolite in 1533. At the time new methods of surveying were being used and by combining an Alidade, a magnetic compass and the degree scale on the back of an Astrolabe, the calculations made by modern theodolites could be observed. Unfortunately, this method was not practical due to the combination of instruments. The best of the attempts to simplify the process was the ‘theodolitus’, first described in print by Leonard Digges in 1571.
However, this instrument could only take measurements in the horizontal plane. Despite this it was still thought of as the ‘common’ Theodolite up to the late 18th century.
During the 19th century the Altazimuth Theodolite was considered the most useful theodolite, as it could measure on the horizontal and vertical planes. Three notable types of Altazimuth Theodolite were developed: The Everest Theodolite, the Plain Theodolite and the Transit Theodolite.
10/03/2009
Created by: Dr. Anita McConnell on 10/03/2009
FM:43637
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