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{{Short description|Surveying instrument for angle measurements}}
[[File:Butterfield graphometer img 2601.jpg|thumb|Butterfield compass graphometer]]
[[File:Göttingen-Astrolabium.mit.Diopter.und.Kompass.JPG|thumb|A German graphometer in Göttingen, Stadtmuseum. The instrument is on its side. At the back, the socket for a [[Jacob's staff]] can be seen.]]
The '''graphometer''', '''semicircle''' or '''semicircumferentor''' is a [[surveying]] instrument used for [[angle]] measurements. It consists of a semicircular [[wiktionary:Limb#Etymology 2|limb]] divided into 180 degrees and sometimes subdivided into minutes. The limb is subtended by the diameter with two [[sight (device)|sights]] at its ends. In the middle of the diameter a "box and needle" ([[compass]]) is fixed. On the same middle the [[alidade]] with two other sights is fitted. The device is mounted on a [[Jacob's staff#Surveying|staff]] via a [[ball and socket]] joint. In effect the device is a half-[[circumferentor]]. For convenience, sometimes another half-circle from 180 to 360 degrees may be [[graduation (instrument)|graduated]] in another line on the limb.<ref name=c1728>{{1728|title=Graphometer}} Pages: vol. 1 p. 179 for "Graphometer", vol. 2 p. 50 for "Semi-Circle"</ref>

The form was introduced in [[Philippe Danfrie]]'s {{Lang|fr|Déclaration de l’usage du graphomètre}} (Paris, 1597)<ref name="Danfrie 1597">{{cite book |last1=Danfrie |first1=Philippe |title=Declaration de l'usage du graphomètre |date=1597 |publisher=Philippe Danfrie |location=Paris |url=https://books.google.com/books?id=3rBTAAAAcAAJ}}</ref><ref name="Vervliet2020">{{cite journal |last1=Vervliet |first1=Hendrik D L |author-link=H. D. L. Vervliet|title=Danfrie Reconsidered. Philippe Danfrié's (d. 1606) Civilite Types |journal=The Library |date=March 2020 |volume=21 |issue=1 |pages=3–45 |doi=10.1093/library/21.1.3|doi-access= }}</ref> and the term ''graphometer'' was popular with French [[Geodesist|geodesists]]. The preferable English-language terms were ''semicircle'' or ''semicircumferentor''. Some 19th-century graphometers had [[telescope|telescopic]] rather than open sights.<ref>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"] {{webarchive|url=https://web.archive.org/web/20071015125250/http://americanhistory.si.edu/collections/surveying/type.cfm?typeid=10 |date=2007-10-15 }} article of the [[Smithsonian]]</ref>
[[File:Grafometro 1856.png|thumb|19th century graphometer]]

[[André Le Nôtre|Le Nôtre]]'s {{Lang|fr|La theorie et la pratique du jardinage}} ('The theory and practice of gardening'), published in 1709, described the use of the graphometer in transferring geometric shapes from garden plans onto landscapes at a large scale.

==Usage==
[[Image:Open angle with letters.svg|thumb|right|Figure 1: Angle EKG]]
To measure an angle, say, EKG, the diameter middle C is placed at the angle apex K using the [[Plumb-bob|plummet]] at point C of the instrument. The diameter is aligned with leg KE of the angle using the sights at the ends of the diameter. The alidade is aligned with the leg KG using another pair of sights, and the angle read off the limb as marked by the alidade. Further uses of the graphometer are the same as those of the circumferentor.<ref name=c1728/>

==References==
<references/>
* Ralf Kern: Wissenschaftliche Instrumente in ihrer Zeit. Vom 15. – 19. Jahrhundert. Verlag der Buchhandlung Walther König 2010, {{ISBN|978-3-86560-772-0}}
{{Commons category|Graphometers}}

[[Category:Angle measuring instruments]]
[[Category:Surveying instruments]]

Graphometer - Revision history

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