It is an arc attached to the rear part of the cheek of a gun-carriage, having its centre in the axis of the trunnions; the arc is graduated into degrees and parts of a degree. By placing the axis of the piece horizontal, and marking the breech at any one of the divisions on the arc, any elevation or depression required will be noted by the number of degrees below or above this mark. It turns on a pivot which admits of the arc, when not in use, being placed inside the cheek to which it is attached. 43. What is the use of the knob of the cascable? To facilitate the handling of the piece in mounting and dismounting it, and moving it when off its carriage. 44. Of what use are the trunnions of a piece? By means of them the piece is attached to its carriage; and by being placed near the centre of gravity, it is easily elevated or depressed. 45. What are the dolphins of a piece? Two handles placed upon the piece with their centres over the centre of gravity, by which it is mounted or dismounted. 46. Are all pieces provided with dolphins? Only the 12-pdr. brass guns, and the 24 and 32-pdr. brass howitzers. 47. What is understood by the preponderance of a piece? It is the excess of weight of the part in rear of the trunnions over that in front; it is measured by the weight which it is necessary to apply in the plane of the muzzle to balance the gun when suspended freely on the axis of the trunnions. 48. Why is this preponderance given? To prevent the sudden dipping of the muzzle, in firing, and violent concussion on the carriage at the breech. [Sidenote: 20]] 49. What is bushing a piece of artillery? Inserting a piece of metal about an inch in diameter (near the bottom of the bore), through the centre of which the vent has been previously drilled. It is screwed in. 50. What kind of metal is used for bushing bronze pieces? Pure copper always, which is not so liable to run from heat as gun metal. 51. What is the object of bushing a piece? To prevent deterioration of the vent, or provide a new one when this has already occurred. 52. Is all new artillery bushed? No, only bronze pieces, and iron pieces, only when repeated firing has rendered it absolutely necessary. 53. How is artillery rendered unserviceable?