in geometry, is a part of a circle comprehend- ed between two radii and the arch; or it is a mixed triangle, formed by two radii and the arch of a circle.

SECTOR is also a mathematical instrument, of great use in finding the proportion between quantities of the same kind, as between lines and lines, surfaces and surfaces, &c., for which reason the French call it the compass of proportion.

The great advantage of the sector above common scale, &c. is, that it is adapted to all radii, and all scales. For, by the line of chords, sines, tangents, &c. on the sector, we have lines of chords, sines, tangents, &c. adapted to any radius betwixt the length and breadth of the sector, when opened.

The sector is founded on the fourth proposition of the sixth book of Euclid, where it is demonstrated, that similar triangles have their homologous sides proportional.