Accession No

0701

Brief Description

theodolite and protractor, by Watkins, English, 1770 (c)

Origin

England; London; Charing Cross

Maker

Watkins

Class

surveying

Earliest Date

1770

Latest Date

1770

Inscription Date

Material

metal (brass); glass; wood

Dimensions

box length 154mm; breadth 129mm; height 114mm

Special Collection

Robert Whipple collection

Provenance

Purchased by Robert Stewart Whipple from T.H. Court on 02/03/1933.

Inscription

‘Watkins Charing Cross LONDON’ (on horizontal circle)
‘Watkins London’ (on vernier arm of protractor)

Description Notes

Small brass theodolite. 2 parts.
1. Telescope with sliding eyepiece focus; inverting; cross hairs. Bubble mounted over telescope. No graduations. ‘A’ frame where the axis supports two parallel arms with linking Y supposrts for telescope. Clip and pin fastenings. Vertical pierced semi-circle controlled by rack and pinion with milled screw on cogged outer edge. Graduated 85 - 0 - 60˚ and read by vernier scale to 3’ of arc. Compass mounted centrally; 8 point silvered rose graduated 0 - 360˚ subdivided to 1; locking bar. Horizontal circle with silvered scale graduated 0 - 360˚ on bevelled edge. Vernier plate reads to 3’ of arc. Motion screw for vernier plate. Clamp for horizontal circle. Central screw thread for attachment to tripod.
2. Brass protractor. Graduated 0 - 360˚. Read by vernier arm to 3’. Suspension point. Cross etched on central glass circle for sighting.
Fitted wooden box.

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:42923