Military Compasses

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Name used here (the inventor assigned it no specific name) to indicate the instrument's chief application.



Carlo Teti

Historic Period



Described by Carlo Teti (1575) as an instrument of military use, this compass was a forerunner of the better-known geometric and military compasses of Galileo. The legs are 11/4 palms long, so that when opened they form a ruler one braccio long, serving as unit of measurement; engraved on their sides are various scales and units of measurement such as the palm and the Roman foot. Two arms pivoting on the legs serve to lock the compass setting at right angles to form a gunner's square or a quadrant. Engraved on the sides of these arms are the degree scale and the shadow square. The curved points serve to measure calibers and, through the relevant weight scale, to calculate the weight of cannonballs. Other operations described by the author include dividing angles and circumferences into equal parts, reading diurnal and nocturnal hours, surveying the layouts of buildings and territories (using special sights mounted on the compass legs), determining direction in excavating mines and tunnels, levelling planes and "other things, which I will omit insofar as they are not truly necessary ...".

Bibliographical Resources

Teti, Carlo. Discorsi delle fortificationi, del sig. Carlo Tetti, ove diffusamente si dimostra, quali debbano essere i siti delle fortezze... et altre cose a loro appartenenti, con le figure di esse, Venetia, appresso Bolognino Zaltiero, 1575.

Existing Instruments

- Museo Galileo, Institute and Museum of the History of Science, Florence
Florence, Museo Galileo. Institute and Museum of the History of Science, inv. 3692.
Florence, Museo Galileo. Institute and Museum of the History of Science, inv. 1277.


Author of the entry: Filippo Camerota

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