Accession No

1395

Brief Description

9-inch transit theodolite, by Thomas Jones and Sons, English, c. 1840

Origin

62 Charing Cross; London

Maker

Thomas Jones & Sons

Class

surveying

Earliest Date

1840

Latest Date

1840

Inscription Date

Material

metal (brass); glass

Dimensions

height 460mm; length 310mm; breadth 235mm

Special Collection

Wollaston collection (??)

Provenance

Presumed transferred from the Cavendish Laboratory, University of Cambridge.

Inscription

‘Thomas Jones and Sons 62 Charing Cross LONDON’

Description Notes

Transit theodolite, by Thomas Jones and Sons, English, c. 1840.

Brass throughout. Inverting telescope. Eyepiece tube focussed by rack and pinion (screw missing) and fits in a slide permitting slight lateral motion of the eyepiece. Lens cover. Bubble mounted on the vertical vernier circle. Ivory strip mounted over bubble graduated to 2’. Vertical silvered scale divided 0 - 360˚, subdivided to 10’ of arc; fixed to axis and telescope and rotates with them. Read by 2 verniers, mounted on spoked circle fixed rigidly to telescope support, to 1’ of arc. 2 microscopes on arms and brackets attached to to support (one lens missing). Clipping screws. Axis supported in’Y’s at the top of the pillar supports and kept in place by metal straps. Axis is hollow and one end glazed to allow illumination of telescope, Bubble mounted on supports with ivory scale over supports mounted on vernier plate. Horizontal silvered scale divided 0 - 360 subdivided to 10’ of arc. Read by 2 verniers and microscopes to 1’ of arc. Clamp and motion screws for horizontal circle and vernier plate. (Those for vertical circle are missing. Originally operated on flange on the axis and mounted on a bracket attached to the pillar support). 3 levelling feet fixed to a framed stand of 2 parallel circles below the horizontal circle.

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:44329