It is a tangent scale, the graduations of which are the tangents of each quarter of a degree of elevation, to a radius equal to the distance between the muzzle-sight of the piece, and the axis of vibration of the hausse, which is one inch in rear of the base-ring. At the lower end of the scale is a brass bulb filled with lead. The _slider_ which marks the divisions on the scale is of thin brass, and is clamped at any desired division on the scale by means of a screw. The scale passes through a slit in a piece of steel, with which it is connected by a screw, forming a pivot on which the scale can vibrate laterally. This piece of steel terminates in pivots, by means of which the pendulum is supported on the _seat_ attached to the gun, and is at liberty to vibrate in the direction of the axis of the piece. The _seat_ is of metal, and is fastened to the base of the breech by screws, so that the centres of the steel pivots of vibration shall be at a distance from the axis of the piece equal to the radius of the base-ring. [Sidenote: 18]] A MUZZLE-SIGHT of iron is screwed into the swell of the muzzle of guns, or into the middle of the muzzle-ring of howitzers. The height of this sight is equal to the dispart of the piece, so that a line joining the muzzle-sight and the pivot of the tangent scale is parallel to the axis of the piece. 38. What is a _gunner’s level_, or gunner’s perpendicular? An instrument made of sheet-brass; the lower part is cut in the form of a crescent, the points of which are made of steel; a small spirit-level is fastened to one side of the plate, parallel to the line joining the points of the crescent, and a slider is fastened to the same side of the plate, perpendicular to the axis of the level. 39. What is it used for? To mark the points of sight on pieces. 40. What is a _plummet_? A simple _line_ and _bob_ for pointing mortars. 41. What is a _gunner’s quadrant_? It is a graduated quarter of a circle of sheet-brass, attached to a brass rule 18 inches long. It has a vernier turning on a pivot, to which is attached a spirit-level. To get a required elevation, the vernier is fixed at the indicated degree, the brass rule is then inserted in the bore parallel to the axis of the piece; the gun is then elevated or depressed until the level is horizontal. There is another graduated _quadrant_ of wood, of 6 inches radius, attached to a rule 23.5 inches long. It has a _plumb-line_ and _bob_, which are carried, when not in use, in a hole in the end of the rule, covered by a brass plate. [Sidenote: 19]] 42. What is an _elevating arc_, and its use?