For the purpose of taking an ail dude or a.zimuth circle is used. The former is usually attached to the top of the binnacle cover, or to some form of dumb compass (a metal plate marked with compass points) ; it consists of a pivoted bar carrying sights, and by comparing, the dumb-compass bearing with the compass heading of the ship, the compass bearing of an object may be obtained. The azi muth circle fits over the top of the compass bowl, and has reflecting mirrors :ind sighting wires; by their means the bearing of the sun or other object is ascertained directly from the compass. \Viten the bearing of an object is taken from tlie north or south point. its angular distance from true north or south is called its azimuth. Thus. if an object bears northeast. its azimuth is N. 45' E.
The early history of the compass is involved in more or less obscurity. In a rough form it was known to the Chinese at least as early as me. 3634, and it was used for the purposes of navigation as early as the third or fourth century A.D., and haps before. But the poliev of the rulers and the habits and character of the people con spired to render the Chinese in different navigators, and the compass did not, therefore, be come of the great importance to them that it did to the sea faring nations of Europe. The date of introduction of the magnetic needle into Europe is unknown; but if it came, as many suppose. from the Chi nese through the Arab sailors and traders, it probably was already a nautical instrument. The first reference to it in liter ature is in a work by Alexan der Neekam. entitled Dc (Tien silibtH, and written in the twelfth century. lle refers to
it as a needle which is placed on a pivot. and when allowed to come to rest shows the ner the direction to steer. In another work. De Naluris rum (lib. ii., c. 80), he writes as follows: "Mariners at sea. when, through cloudy weather in the day, which hides the sun, or through the darkness of the night, they lose the knowledge of the ter of the world to which they are sailing, touch a needle with a magnet. which Will turn round till, on its motion ceasing, its point will be directed toward the (Chappell. ture, No. 346. June 15, 1876). Subsequent to this there are repeated references to the use of a magnetic needle for navigating purposes. As early as the thirteenth century it seems to have been known to the navigators of all European nations, and in 1269 its declination (or lion from the true north) seems to have been observed. The division of the compass-card into thirty-two points is a natural one, and was merely continuing the subdivision until it be came suffieiently minute. It seems to have been adopted early. as Chaucer writes of it as estab lished in 1391.
BIBLIOGRAPHY. Consult: The Admiralty ManBibliography. Consult: The Admiralty Man- ual for the Deviations of the Compass, by Capt. F. .1. Evans, R.N., F.R.S., and Archibald Smith, F.R.S. (new editions appear at short intervals) : Naral Professional Papers of the United .Stays Nary, No. 22 (Washington, lS96); Cornwell, Compass Disturbance in Iron ylaips (18S7): Bowditeh, The American Prae lira/ .Varipator (Washington. 1901) : publica tions of the Superintendent of Compasses, 'Unit ed States Navy.