is a part of a circle comprehended between two radii and an arc of the circle. Sector is also a mathematical instrument, of use in finding the proportion between quantities of the same kind; as between lines and lines, surfaces and surfaces, &c., whence the French call it the compass of proportion. The great advantage of the sector above the common scales is, that it is made so as to fit all radii, and all scales. By the lines of chords, sines, &c., upon the sector, we have lines of chords, sines, &c., to any radius between the length and breadth of the sector when open. The sector is supposed to have been invented by Guido Baldo, or Ubald, about the year 1568. The first printed account of it was in 1584, by Gaspar Mordente, at Antwerp, who says that his brother, Fabricius Mordente, invented it in 1554. Treatises on its use have been written by Daniel Speele at Strasbourg, in 1589; also by Thomas Hood at London, in 1598; and by Lewin Hulse at Frankfort-on-the-Maine, in 1603, who says that it was invented long before by Justus Brygus. But the honour of the invention was claimed by Galileo, who wrote on its use in 1607, and by Balthasar Capra of Milan. There are also treatises on it by our countrymen Gunter, Forster, and others. Before the invention of logarithms, practical men were more easily contented with approximate solutions than they are at present. Now, however, the sector is not much used, although it is commonly reckoned one of a complete set of mathematical instruments.

For treatises on its use, see Bion on Mathematical Instruments, translated by Stone; Robertson's Treatise on Mathematical Instruments; and Adam's Geometrical Essays.

Any one possessing a sector will easily understand its theory and use from the 14th problem of the fifth section of our treatise on Geometry, where it is taught how to find a fourth proportional to three given lines.

The Astronomical Sector, or Equatorial Sector, is an instrument for taking the difference of right ascensions and declinations of such stars as, on account of their great difference of declinations, will not pass through a fixed telescope.

The Zenith Sector is an instrument employed in extensive trigonometrical surveys. Its use is to determine with great accuracy the zenith distances of stars whose declinations differ but little from the latitude of an observer.