Pieces rejected on inspection are marked X C on the face of the muzzle; if condemned for erroneous dimensions which cannot be remedied, add X D; if by powder proof, X P; if by water proof, X W. 59. What are the kinds of proof which artillery must undergo, before being received into the service? 1st. They are gauged as to their several dimensions, internal and external; as to justness and position of the bore, the chamber, vent, trunnions, &c. 2d. They are fired with a regulated charge of powder and shot, being afterwards searched to discover irregularities or holes produced by the firing. 3d. By means of engines, an endeavor is made to force water through them. 4th. They are examined internally, by means of light reflected from a mirror. [Sidenote: 23]] 60. Are brass cannon liable to external injury, caused by service? They are little subject to such injury, except from the bending of the trunnions sometimes, after long service or heavy charges. =NOTE.=—Recent experiments at Fort Monroe show that brass guns, when _rifled_, and fired with large charges and heavy shot, expand so much that the projectile does not take the grooves. 61. What are the causes of internal injury? Internal injuries are caused by the action of the elastic fluids developed in the combustion of the powder, or by the action of the shot in passing out of the bore. 62. Name the injuries of the first kind. _Enlargement of the bore_ by the compression of the metal; _corrosion of the metal_ at the inner orifice of the vent, or at the mouth of the cylindrical chamber; _cracks_, from the yielding of the cohesion of the metal; _cavities_, cracks enlarged by the action of the gas, and by the melting of the metal, observable especially in the upper surface of the bore. 63. Name those of the second kind. [Sidenote: 24]] The _lodgment of the shot_,—a compression of the metal on the lower side of the bore, at the seat of the shot, which is caused by the pressure of the gas in escaping over the top of the shot. There is a corresponding _burr_ in front of the lodgment; and the motion thereby given to the shot causes it to strike alternately on the top and bottom of the bore, producing other _enlargements_, generally _three_ in number: the first, on the upper side a little in advance of the trunnions; the second, on the lower side about the astragal; the third, in the upper part of the muzzle; it is chiefly from this cause that brass guns become unserviceable. _Scratches_, caused by the fragments of a broken shot, or the roughness of an imperfect one; _enlargement of the muzzle_ by the striking of the shot in leaving the bore; _external cracks_, or longitudinal slits, caused by too great a compression of the metal on the inside. 64. When is a piece said to be honeycombed? When the surface of the bore is full of small holes or cavities. 65. To what is this due?