Drawing of a circumferentor from the Cyclopaedia

A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles, now superseded by the theodolite. 1913 advertisement for Encyclopædia Britannica. ... Surveyor at work with a leveling instrument. ... Diagram of an Optical Theodolite. ...

It consists of a brass circle and an index, all of one piece. On the circle is a card, or compass, divided into 360 degrees; the meridian line of which is in the middle of the breadth of the index. On the circumference of the circle is a brass ring, which, with another ring fitted with glass, make a kind of box for the needle, which is suspended on a rivet in the center of the circle. On each extreme of the index is a sight. The whole apparatus is mounted on a staff, with a ball-and-socket joint for easy rotation. Compass in a wooden box A compass (or mariners compass) is a navigational instrument for finding directions on the earth. ... The Prime Meridian, Greenwich The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. ... Sight of a Sig 550 rifle (muzzle) Sight of a Sig 550 rifle (stock) A sight is an optical device used to assist aim by guiding the eye and aligning it with the weapon or other item to be pointed. ... A joint (articulation) is the location at which two bones make contact (articulate). ...

Circumferentors were made throughout Europe, including England, France, Italy, and Holland. By the early 1800s, Europeans preferred theodolites to circumferentors. However, in America, and other wooded or uncleared areas, the circumferentor was still in common use. Diagram of an Optical Theodolite. ...

Usage

Figure 1: Angle EKG

Measuring angles

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. Fleurs-de-lys on the flag of Quebec The fleur-de-lis (also spelled fleur-de-lys; plural fleurs-de-lis or -lys) is used in heraldry, where it is particularly associated with the France monarchy (see King of France). ...

If the remainder is more than 180 degrees, it must be subtracted from 360 degrees.

Surveying a region

To take the plot of a field, forest, park, etc, with a circumferentor, consider region ABCDEFGHK in Figure 2, an area to be surveyed.

1. Placing the instrument at A, the fleur-de-lis towards you, direct the sights to B; where suppose the south end of the needle cuts 191°; and the ditch, wall, or hedge, measuring with a Gunter's chain, contains 10 chains, 75 links.
2. Placing the instrument at B, direct the sights as before to C; the south end of the needle, e.g. will cut 279°; and the line BC contains 6 chains and 83 links.

Then move the instrument to C; turn the sights to measure D, and measure CD as before. In the same manner, proceed to D, E, F, G, H, and lastly to K; still noting the degrees of every bearing, or angle, and the distances of every side. This will result in a table of the following form: Edmund Gunter (1581 - December 10, 1626), English mathematician, of Welsh extraction, was born in Hertfordshire in 1581. ...

From this table, the field is to be plotted, or protracted.

References

• This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.
• Gerard L'E Turner. "Circumferentor". Scientific Instruments, 1500-1900. University of California Press. ISBN 0520217284. 1998.
• "Circumferentor". Oxford English Dictionary. Oxford University Press. 2nd edition. 1989.