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Reduction Compass
circa 1600; Italian
Brass and steel; 181 mm in radius

The instrument consists of two jointed brass legs which open to a maximum of 180?. At the end of each leg is a steel point so that, when used vertically, the instrument serves as a pair of dividers. One point has a small projecting pin and the other a socket so that they are mated together when the instrument is closed.

The instrument is however principally intended to be used horizontally, and it is supported above a drawing surface by steel points perpendicular to the legs. A point projects on either side of the joint, providing a pair of fixed points. The remaining points are mounted on the legs and their position can be adjusted and fixed with screws. Each leg has a plain point and there is also a point offset from its sliding sleeve. The remaining sliding piece is a more complex assembly: there is an offset point paired with a sliding point (arranged together in the manner of a caliper), and these are coupled with a point which can be used to make pin pricks. The latter is arranged as a sliding rod with a decorated head and a slim steel point held in an upright. By pushing down on the head of the rod a mark could be made where desired on the paper.

No scales are required on a reduction compass, and this example also lacks any indication of maker, origin or date.

Stephen Johnston

Museum of the History of Science, Oxford
Inventory number 49857

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