Altazimuth theodolite

Altazimuth theodolite made by John Bennett.

Altazimuth theodolites are instruments for measuring both altitude and azimuth, i.e. vertical and horizontal angles.

This one belongs to the collection of the scientific instruments of King George III. The horizontal circle at the base is graduated in degrees, with every tenth degree numbered. The vertical arc is graduated to 54° on either side of zero and supports the telescope. The spirit level that would originally have been mounted above the telescope is now missing. The instrument could be used directly on a plane table, allowing surveyors to plot measurements immediately onto a map or plan. It could also be mounted on a tripod, although no tripod fitting is visible, suggesting that this component may also have been lost.

Theodolites were widely used for surveying from the early seventeenth century in Europe and, to a lesser extent, in North America. Around 1720, the London instrument maker Jonathan Sisson developed the telescopic theodolite, greatly improving the instrument's accuracy and versatility and leading to the widespread adoption of the dual-purpose altazimuth theodolite.