Circumferentor

Surveying tool to measure horizontal angles

A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century. A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees. The needle is protected by a glass covering. A pair of sights is located on the North South axis of the compass. Circumferentors were typically mounted on tripods and rotated on ball and socket joints. Circumferentors were made throughout Europe, including in England, France, Italy, and Holland. By the early 19th century, Europeans preferred theodolites to circumferentors. However, the circumferentor remained in common use in mines and in wooded or uncleared areas, such as in America.

To measure an angle with a circumferentor, such as angle EKG (Figure 1), place the instrument at K, with the fleur de lis in the card towards you. Then direct the sights, until through them you see E; and note the degree pointed at by the south end of the needle, such as 296°. Then, turn the instrument around, with the fleur de lis still towards you, and direct the sights to G; note the degree at which the south end of the needle point, such as 182°. Finally, subtract the lesser number, 182, from the greater, 296°; the remainder, 114°, is the number of degrees in the angle EKG. If the remainder is more than 180 degrees, it must be subtracted from 360 degrees.

A double prism is a device to measure right angles, consisting of two five sided prisms stacked on top of each other and a plumb bob. It is used to stake out right angles, for example on a construction site.