Accession No

0502

Brief Description

altazimuth theodolite, fitted with inverting telescope, by J. Sisson, English, 1740 (c)

Origin

England; London

Maker

Sisson, J.

Class

surveying

Earliest Date

1740

Latest Date

1740

Inscription Date

Material

metal (brass, white metal, silver, steel); glass; liquid

Dimensions

Horizontal circle diameter 355 mm; Telescope length 293mm; height 237mm

Special Collection

Robert Whipple collection

Provenance

Purchased from T.H. Court in 05/1927.

Inscription

‘J. Sisson Fecit’ (on horizontal circle)

Description Notes

Brass altazimuth theodolite, fitted with inverting telescope, by Sisson, c 1740.

Brass theodolite; inverting telescope; sliding eyepiece; cross hairs; sliding lens cover for eyepiece and screw-fit objective (lens missing). Bubble level screws onto mounts over telescope. The whole mounted by two milled screws onto a pierced frame from the central point of the vertical semi-circle. Vertical circle graduated [90] - 0 - [90]° numbered by 10 and subdivided to single degrees and read by vernier mounted on the telescope frame to 3’ of arc. Vertical semi-circle attached by two screws to index moving over horizontal circle. Compass with silvered 8 point rose. Lower scale [0] - [90] - [0]° twice, numbered by 10. Upper scale [0] - 360° numbered by 10, subdivided to single degrees. Steel needle; glazed. Large horizontal circle supported by 6 spokes from central circle. Graduated [0] - 360° numbered by 10 subdivided to 30’ of arc. Read by vernier to 2’ of arc.

Complete

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.

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.

The maker of this particular instrument, Jonathan Sisson (c. 1690–1747), was the first maker of these telescopic theodolites, which were soon copied and improved by other craftsmen.

10/03/2009
Created by: Dr. Anita McConnell on 10/03/2009

FM:40195