Horizontal, a.
Etymology: [Cf. F. horizontal.]
1. Pertaining to, or near, the horizon. "Horizontal misty air." Milton.
2. Parallel to the horizon; on a level; as, a horizontalline or surface.
3. Measured or contained in a plane of the horizon; as, horizontal distance. Horizontal drill, a drilling machine having a horizontal drill spindle. -- Horizontal engine, one the piston of which works horizontally. -- Horizontal fire (Mil.), the fire of ordnance and small arms at point-blank range or at low angles of elevation. -- Horizontal force (Physics), the horizontal component of the earth's magnetic force. -- Horizontal line (Descriptive Geometry & Drawing), a constructive line, either drawn or imagined, which passes through the point of sight, and is the chief line in the projection upon which all verticals are fixed, and upon which all vanishing points are found. -- Horizontal parallax. See under Parallax. -- Horizontal plane (Descriptive Geometry), a plane parallel to the horizon, upon which it is assumed that objects are projected. See Projection. It is upon the horizontal plane that the ground plan of the buildings is supposed to be drawn. -- Horizontal projection, a projection made on a plane parallel to the horizon. -- Horizontal range (Gunnery), the distance in a horizontal plane to which a gun will throw a projectile. -- Horizontal water wheel, a water wheel in which the axis is vertical, the buckets or floats revolving in a horizontal plane, as in most turbines.
Shades of meaning
Horizontal signifies in the direction of or parallel to the horizon. For practical purposes level and horizontal are identical, tho level, as the more popular word, is more loosely used of that which has no especially noticeable elevations or inequalities; as, a level road. Flat, according to its derivation from the Anglo-Saxon flet, a floor, applies to a surface only, and, in the first and most usual sense, to a surface that is horizontal or level in all directions; a line may be level, a floor is flat; flat is also applied in a derived sense to any plane surface without irregularities or elevations, as a picture may be painted on the flat surface of a perpendicular wall. Plane applies only to a surface, and is used with more mathematical exactness than flat. The adjective plain, originally the same word as plane, is now rarely used except in the figurative senses, but the original sense appears in the noun, as we speak of "a wide plain." We speak of a horizontal line, a flat morass, a level road, a plain country, a plane surface (especially in the scientific sense). That which is level may not be even, and that which is even may not be level; a level road may be very rough; a slope may be even.