HORIZONTAL

, something that relates to the Horizon, or that is taken in the Horizon, or on a level with or parallel to it. Thus, we say, a Horizontal plane, Horizontal line, Horizontal distance, &c.

Horizontal Dial, is one drawn on a plane parallel to the horizon; having its gnomon or style elevated according to the altitude of the pole of the place it is designed for.

Horizontal Distance, is that estimated in the direction of the horizon.

Horizontal Line, in Perspective, is a right line drawn through the principal point, parallel to the horizon; or it is the intersection of the Horizontal and perspective planes.

Horizontal Line, or base of a hill, in Surveying, a line drawn on the Horizontal plane of the hill, or that on which it stands.

Horizontal Moon. See Apparent Magnitude.

Horizontal Parallax. See Parallax.

Horizontal Plane, is that which is parallel to the horizon of the place, or not inclined to it.|

Horizontal Plane, in Perspective. See Plane.

Horizontal Projection. See Projection, and Map.

Horizontal Range, of a piece of ordnance, is the distance at which it falls on, or strikes the horizon, or on a Horizontal plane, whatever be the angle of elevation or direction of the piece. When the piece is pointed parallel to the horizon, the range is then called the point-blank or point-blanc range.

The greatest Horizontal range, in the parabolic theory, or in a vacuum, is that made with the piece elevated to 45 degrees, and is equal to double the height from which a heavy body must freely fall to acquire the velocity with which the shot is discharged. Thus, a shot being discharged with the velocity of v feet per second; because gravity generates the velocity 2g or 32 1/6 feet in the first second of time, by falling 16 1/12 or g feet, and because the spaces descended are as the squares of the velocities, therefore as the space a body must descend to acquire the velocity v of the shot or the space due to the velocity v; consequently the double of this, or is the greatest Horizontal range with the velocity v, or at an elevation of 45 degrees; which is nearly half the square of a quarter of the velocity.

In other elevations, the Horizontal range is as the sine of double the angle of elevation; so that, any other elevation being e, it will be, as radius , the range at the elevation e, with the velocity v.

But in a resisting medium, like the atmosphere, the actual ranges fall far short of the above theorems, in so much that with the great velocities, the actual or real ranges may be less than the 10th part of the potential ranges; so that some balls, which actually range but a mile or two, would in vacuo range 20 or 30 miles. And hence also it happens that the elevation of the piece, to shoot farthest in the resisting medium, is always below 45°, and gradually the more below it as the velocity is greater, so that the greater velocities with which balls are discharged from cannon with gunpowder, require an elevation of the gun equal to but about 30°, or even less. And the less the size of the balls is too, the less must this angle of elevation be, to shoot the farthest with a given velocity. See PROJECTILE, and Gunnery.

Horizontal Refraction. See Refraction.

Horizontal Speculum, one to find a horizon at Sea, &c, when the atmosphere is hazy near the horizon, by which the fight of it is prevented.

A speculum of this kind was invented by a Mr. Serson, on the principle of a top spinning, which always keeps its upright position, notwithstanding the motion of the substance it spins upon. This curious instrument, as it has since been improved by Mr. Smeaton, consists of a well polished metal speculum, of about 3 inches and a half in diameter, inclosed within a circular rim of brass; so fitted that the centre of gravity of the whole shall fall near the point on which it spins. This is the end of a steel axis running through the centre of the speculum, above which it finishes in <*> square, for the conveniency of fitting a roller on it, which sets it in motion by means of a piece of tape wound about the roller.

Various other contrivances to form artificial horizon<*> have been invented by different persons, as glass planes floating on mercury, &c. See Hadley's Quadrant, and several inventions of this sort in the Philos. Trans. by Elton, Halley, Leigh, &c. vol. xxxvii, p. 273, vol. xxxviii, p. 167, vol. xl, p. 413, 417, &c.

HORN-Work, in Fortification, a sort of out-work, advancing toward the field, to cover and defend a curtin, bastion, or other place, suspected to be weaker than the rest; as also to possess a height; carrying in the fore-part, or head, two demi-bastions, resembling horns: these horns, epaulments, or shoulderings, being joined by a curtin, shut up on the side by two wings, parallel to one another, are terminated at the gorge of the work, and so present themselves to the enemy.