Circumferentor with mahogany case

This circumferentor was used to measure horizontal angles and bearings between points on the ground. The surveyor sighted along the two arms toward a landmark and read its bearing from the compass dial. A series of these bearings, combined with distances measured by chain, allowed a surveyor to plot the shape of a piece of land.

The vernier scale allowed the surveyor to make more precise angular measurements, while the spirit level helped ensure that the instrument was level when in use. The two scales on the lid were used to correct measurements taken on sloping ground: they showed how many links should be subtracted from a 100-link chain measurement to calculate the true horizontal distance. The reference to links reflects the instrument's intended use alongside a surveyor's chain.

Instruments like this were a common tool for eighteenth-century land surveying, before more precise and increasingly affordable theodolites became widespread. Circumferentors were particularly popular in North America, where land was cheaper, so precision mattered less, and trees often blocked the sightlines a theodolite needed.

This circumferentor is part of King George III's collection of scientific instruments.

This circumferentor was used to measure horizontal angles and bearings between points on the ground. The surveyor sighted along the two arms toward a landmark and read its bearing from the compass dial. A series of these bearings, combined with distances measured by chain, allowed a surveyor to plot the shape of a piece of land.

The vernier scale allowed the surveyor to make more precise angular measurements, while the spirit level helped ensure that the instrument was level when in use. The two scales on the lid were used to correct measurements taken on sloping ground: they showed how many links should be subtracted from a 100-link chain measurement to calculate the true horizontal distance. The reference to links reflects the instrument's intended use alongside a surveyor's chain.

Instruments like this were a common tool for eighteenth-century land surveying, before more precise and increasingly affordable theodolites became widespread. Circumferentors were particularly popular in North America, where land was cheaper, so precision mattered less, and trees often blocked the sightlines a theodolite needed.

This circumferentor is part of King George III's collection of scientific instruments.

The circumferentor's brass cover plate for the dial is marked with two scales. One is degrees 45-0-45 and the other 30-0-30 engraved 'Diff of Hypo and Base'. This refers to the numbers of links of chain needed to be subtracted from a 100-link measurement on an incline to find the distance travelled horizontally.