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An Acute Angle

angles, solid and degrees

AN ACUTE ANGLE is that which is less than one right angle. The angle formed when A B has completed one revo lution and arrived at A 11, is described as four right angles + angle n A n.

Measurement of Angles. Referring again to the last dia gram, it will be seen that the point a in the line A B, during its revolution round its axis A, describes a circle. Now the circumference of any circle so described is supposed to be divided into 360 equal parts, called degrees, each of such degrees into 60 minutes, and each minute into 60 seconds. This division is made use of for the measurement of angles in the following manner :—As the angle traced out by a whole revolution passes over in its progress 360 of the larger divisions, it is styled the angle of 360 degrees ; similarly, the right angle, which makes only a quarter revolution, is named the angle of 90 degrees ; and so on for angles of any dimen sions whatsoever.

The measure of the are is sometimes used indiscriminately for that of the angle ; but such measurement is, strictly speaking, incorrect. See ARC, External ANGLE, in civil architecture, the same as Saliant ANGLE, which see.

Internal ANGLE, in civil architecture, the same as Re entering ANGLE, which see.

Re-entering, or Re-entrant ANGLE OF A SOLID; an angle whose vertex recedes, or is turned inwards, from a right line extended between any two points in the legs; or it is a cavity or void, formed by two planes on the surtlice of the solid. Artificers call all such angles, made by walls or partitions, Internal Angles.

Saliant or Sortant ANGLE OF A SOLID, an angle, of which the vertex is prominent ; or it is the solid matter contained between two planes inclined to each other in an angle less than two right angles ; or, it is such, that if a point be taken in each plane, the straight line joining the two points, will pass through the solidity. Artificers call all such angles, made by walls or partitions, External Angles.

Solid ANGLE, the mutual inclination of more than two plane rectilineal angles meeting in a point, and not contained in the same plane.