Italian-Style Sector

This brass instrument has two hinged arms, each with three curves at the end. Six double scales (i.e., the scales on one arm are identical to those on the other arm) are on one side. The outermost runs from 1 to 12 and is labeled "Forti" on one arm. The second scale is labeled "Lin : Metali :" on the other arm and marked with the letters: I, T, C, S, L, Q, G. These correspond to the specific weights of different metals. The third scale runs from 30 to 3 and is labeled "Div Cir" on the first arm. The fourth scale runs from 10 to 180 and is not labeled. The fifth scale runs from 5 to 100. The sixth scale is located where the two arms meet and is marked with engravings of various polyhedra. Two short single scales are on the lower arm near the hinge; one is marked L and the other is marked I.

The reverse side also has six double scales. The outermost is marked with polyhedra. The second runs from 3 to 30 and is labeled sub poli on one arm. The third is a scale of equal parts that runs from 10 to 200. The fourth runs from 15 to 3 and is labeled Tetrag : on one arm. This is a scale of tetragonic lines, for squaring regular areas and the circle. The fifth runs from 10 to 100 and is labeled Qua on one arm. The innermost scale is marked 1/6 of the distance from the hinge with a 6, 1/5 of the way with a 5, and so on to halfway down the instrument with a 2. The scale is labeled "Divid :" on the lower arm.

Galileo outlined this arrangement of scales in 1606, which is why this example is considered to be in the Italian style. The previous owner believed it was made in the 18th century. The inside edge of one arm is marked with the letters I S, but nothing else is known about the maker.

Reference: Galileo Galilei, Operations of the Geometric and Military Compass, trans. and intro. Stillman Drake (Washington, D.C.: Smithsonian Institution Press, 1978).

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