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The graphometer, semicircle or semicircumferentor is a surveying instrument used for
"Graphometer"
article of the Smithsonian
Le Nôtre's ('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.
The graphometer, semicircle or semicircumferentor is a surveying instrument used for angle
In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the '' vertex'' of the angle.
Angles formed by two rays lie in the plane that contains the rays. Angles a ...
measurements. It consists of a semicircular limb
Limb may refer to:
Science and technology
*Limb (anatomy), an appendage of a human or animal
*Limb, a large or main branch of a tree
*Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb
*Limb, in botany, ...
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" (compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself wit ...
) is fixed. On the same middle the alidade
An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to triangulate a scale map on site ...
with two other sights is fitted. The device is mounted on a staff via a ball and socket
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
joint. In effect the device is a half-circumferentor
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 It was superseded by the theodolite in the early 19th century.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 n ...
. 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
Philippe Danfrie the elder (about 1532 in Cornouaille in Brittany - 1606 in Paris), was a designer and maker of mathematical instruments in metal and paper, as well as a type-cutter, engraver, minter of coins and medals, publisher and author. Much ...
's (Paris, 1597) and the term ''graphometer'' was popular with French geodesists. The preferable English-language terms were ''semicircle'' or ''semicircumferentor''. Some 19th-century graphometers had telescopic rather than open sights.J. A. Bennett, "The Divided Circle" (Oxford, 1987), pp. 49-50, as quoted in th"Graphometer"
article of the Smithsonian
Le Nôtre's ('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
To measure an angle, say, EKG, the diameter middle C is placed at the angle apex K using the 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.References