- Graphometer
Graphometer or semicircle is a
surveying instrument used forangle measurements. It consists of a semicircular divided into 180 degrees and sometimes subdivided into minutes. The limb is subtended by the diameter with twosight s at its ends. In the middle of the diameter a "box and needle " (acompass ) is fixed. On the same middle thealidade with two other sights is fitted. The device is mounted on a staff via aball and socket joint. In effect the device is a half-circumferentor . For convenience, sometimes another half-circle from 180 to 360 degrees may be graduated in another line on the limb. 1728 Pages: vol. 1 p. 179 for "Graphometer", vol. 2 p. 50 for "Semi-Circle" ]The form was introduced in Philippe Danfrie's, "Déclaration de l’usage du graphomètre" (Paris, 1597) and the term "graphometer" was popular with French geodesists. The preferable English language terms were semi-circle or semicircumferentors. Some 19th-century graphometers had telescopic rather than open sights. [J. A. Bennett, "The Divided Circle" (Oxford, 1987), pp. 49-50, as quoted in the [http://americanhistory.si.edu/collections/surveying/type.cfm?typeid=10 "Graphometer"] article of the
Smithsonian ]Usage
To measure an angle, say, EKG, place the diameter middle C at the angle apex K using the
plummet at point C of the instrument. Align the diameter with leg KE of the angle using the sights at the ends of the diameter. Align the alidade with the leg KG using another pair of sights, and read the angle off the limb as marked by the alidade. Further uses of the graphometer are the same as those of the circumferentor.References
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