Accession No
1464
Brief Description
7-inch plain theodolite, by Fraser and Son, English, 1810 (c)
Origin
England; London
Maker
Fraser and Son
Class
surveying
Earliest Date
1810
Latest Date
1810
Inscription Date
Material
metal (brass); glass
Dimensions
length 397mm; height 222mm; base diameter 190mm
Special Collection
Provenance
Transferred from the Department of Geography, University of Cambridge in 05/1969.
Inscription
‘Fraser and Son. Mathematl. Inst. Maker to his Majesty and the Honble. Board of Ordnance. LONDON’ (on horizontal circle)
Description Notes
7-inch plain theodolite, by Fraser and Son, c. 1810.
Brass throughout. Erecting telescope with cross hairs, rack and pinion focus moving objective and sliding eyepiece focus. Bubble mounted over telescope; no graduations. Telescope mounted in ‘Y’ bearings (one pin missing), over the diameter of the vertical pierced semi-circle. Silvered scale divided 50˚ - 0˚ - 90˚ subdivided to 30’; vernier scale. On reverse two scales: ‘Diff. of Hypo and Base’ divided 30 - 0 -30 subdivided to 1; and ‘Perp in 100 of base’ divided 100 - 0 - 100 subdivided to 1. Cogged outer edge of vertical circle controlled by pinion with milled screw. Compass divided 0 - 90˚ - 0 twice, and 0 - 360˚. 2 bubble levels originally mounted on horizontal circle (one bubble level missing, other dry). Horizontal circle with bevelled edge calibrated 0 - 360˚, subdivided to 30’; vernier scale. Vernier plate controlled by rack and pinion motion screw.
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.
This is a fine example of a precision theodolite, with finely-divided silvered scales and all movements controlled by milled screws. The erecting telescope can be raised to a point up to 50º above the horizon; the base, with compass and bubble levels, is rotated to measure horizontal angles. The graduated vertical and horizontal scales are silvered, with a vernier on the horizontal scale.
William Fraser (d.1812) and his son held royal appointments to King George III and to the Board of Ordnance.
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:42920
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