AMPLITUDE. As a general term, implies extent. In astronomy, it is an arc of the horizon intercepted between the true east or west points thereof, and the centre of the sun, star, or planet, at its rising or setting. In other words, it is the horizontal angular distance of a star from the east or west points. It is eastern or ortive when the heavenly object rises, and western or occiduous when it sets, and is moreover northern or southern according to its quarter of the horizon.--_Amplitude_, in gunnery, is the range or whole distance of a projectile, or the right horizontal line subtending the curvilineal path in which it moved.--_Amplitude_, in magnetism, is the difference between the rising and setting of the sun from the east and west points, as indicated by the mariner's or magnetic compass--which subtracted from the true amplitude, constitutes the error of the compass, which is the combined effect of variation and local deviation. AMPOTIS. The recess or ebb of the tide. AMRELL. An archaic orthography for _admiral_. AMULET. A small relic or sacred sentence, preservative against disaster and disease, appended to the neck by superstitious people: few Italian or Spanish seamen are without them. AMUSETTE. A kind of gun on a stock, like that of a musket, but mounted as a swivel, carrying a ball from half a pound to two pounds weight. AMY. A foreigner serving on board, subject to some prince in friendship with us. ANACLASTICS, OR ANACLATICS. The ancient doctrine of refracted light or dioptrics.--_Anaclastic curves_, the apparent curves formed at the bottom of a vessel full of water, or anything at great depths overboard to an eye placed in the air; also the heavenly vault as seen through the atmosphere. ANADROMOUS. A term applied to migratory fishes, which have their stated times of ascending rivers from the sea, and returning again, as the salmon and others. ANALEM. A mathematical instrument for finding the course and elevation of the sun. ANALEMMA. A projection of the sphere on the plane of the meridian, taken in a lateral point of view, so that the colours become circles, whilst those whose planes pass through the eye become right lines, and the oblique circles ellipses. On globes it is represented by a narrow double-looped formed figure, the length of which is equal to the breadth of the torrid zone, and is divided into months and days, to show approximately the solar declination and the equation of time. ANALOGY. Resemblance, relation, or equality; a similitude of ratios or proportions.