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Graphometer

    '''Graphometer''' or '''semicircle''' is a surveying instrument used for angle measurements It consists of a semicircular limb divided into 180 degrees and sometimes subdivided into minutes The limb is subtended by the diameter with two sights at its ends In the middle of the diameter a "box and needle" (a compass) is fixed On the same middle the alidade with two other sights is fitted The device is mounted on a staff via ball and socket 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 Pages: vol 1 p. 179 for "Graphometer" vol 2 p. 50 for "Semi-Circle"
    The form was introduced in Philippe Danfrie Déclaration de l’usage du graphomètre (Paris 1597) and the term "graphometer" was popular with French geodesists The preferrable English language terms were semi-circle or semicircumferentors Some 19th-century graphometers had telescopes rather than open sights J A. Bennett "The Divided Circle" (Oxford 1987) pp. 49-50 as quoted in the "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