Accession No

1588

Brief Description

small, plain brass theodolite, by J. Sisson, England, 1750 (c)

Origin

England; London

Maker

Sisson, J.

Class

surveying

Earliest Date

1750

Latest Date

1750

Inscription Date

Material

metal (brass), glass

Dimensions

Horizontal circle 100 mm; Telescope length 119 mm; height 120 mm

Special Collection

Provenance

Donated in 06/1971.

Inscription

‘J. SISSON LONDON’ (on horizonal vernier arm)

Description Notes

Small plain brass theodolite, by J. Sisson, England, c. 1750.

Inverting telescope; sliding eyepiece; sliding objective cover and ray shade; cross hairs; bubble slung below telescope. Mounted in Y bearings with clip and pin fastenings. Vertical, pierced semicircle, graduated 65 - 0 - 65° in single degrees. Read by vernier to 6’ of arc. Moved manually. Central milled screw clamp for vertical motion. Horizontal circle with 3 spokes. Calibrated 0 - 180° twice on flat edge, in single degrees. Read by single vernier to 6’ of arc. Beneath the circle compass graduated 20° either side of N (no points). 4 levelling feet through parallel plate with central screw for attachment to tripod.

Good condition, complete.

References

Events

Description
The Theodolite is a relatively, simple tool used for measuring angles, both horizontal and vertical. They work using the same principles as a protractor, the ‘point A’ is located and the angle noted, and then the telescope is pointed at ‘point B’ and the second angle is taken.

Although primarily used in surveying the theodolite can be applied to both Meteorology and Navigation.

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. It is the Transit Theodolite, which is still used today.

FM:39510