ARTIFICIAL HORIZON, a contrivance of great utility for enabling an observer to determine the altitude of a heavenly body, or of a terrestrial object, above the horizon of any place, when the sensible horizon is ill defined. The surface of a fluid not easily disturbed by the air, such as quicksilver, or some viscous, opaque fluid, is usually employed for this purpose, as they will adjust themselves to a plane parallel to the rational horizon. To prevent the influence of winds in the open air, the surface is usually covered by a plate of ground-glass with parallel surfaces. In fixed observations this is not necessary; and the mercury is contained in an oblong trough: for locomotive observations, a cup of three inches in diameter is the containing vessel. But as carrying about mercury is inconvenient, some have employed polished metallic or glass mirrors, adjusted by screws at the corners, and a spirit-level, to horizontality; though this is less accurate than the fluid surface. The surface of the quicksilver, or mirror so adjusted, is a plane touching the surface of the earth where the observation is made, and parallel to the rational horizon; therefore, a ray of light passing from the object to the surface of the instrument, forms an angle with that surface equal to the angular elevation of that object above the true horizon of the place, when it is corrected for parallax and refraction. (T. S. T.)