Royals, or Coehorn mortars, having a hole .1 of an inch in depth in the chamber, or .15 in the chase, are rejected: royal howitzers the same. 8 inch howitzers having a hole .15 of an inch in depth in the chamber, or .2 in the chase, will be rejected. _Carronades._—The diameter and position of their bore and chamber must not deviate ¹/₂₀ of an inch. They are proved with two rounds, with their chambers full of powder, and 1 shot and 1 wad. A hole of ²/₁₀ of an inch in depth in the bore, or ¹/₁₀ in the chamber condemns the piece. _Proof Charges._ +--------+----+----+----+----+----+ |68 Prs. | 42 | 32 | 24 | 18 | 12 | +--------+----+----+----+----+----+ |13 lbs. | 9 | 8 | 6 | 4 | 3 | +--------+----+----+----+----+----+ All ordnance, after having undergone this proof, and the subsequent searching, are subject to the water proof: this is done by means of a forcing pump, having a pipe or hose fixed to the mouth of the piece: after two or three efforts to force the water through any honeycombs or flaws which may be in the bore, they are left to dry; and generally the next day examined by the reflected light from a mirror. If the bore contains any small holes or flaws which have not been discovered by the former proofs, they are very readily found by this; the water will continue to weep, or run from the holes, when the solid parts of the bore are perfectly dry. Ordnance suspected of being bad are often subject to a more severe proof: that of firing 30 rounds quick, with the service charge and 2 shot; and in doubtful cases, where the purity of the metal is suspected, recourse has been had to chemical trials and analysis. A quantity of clean filings taken from a part of an iron gun free from rust, are dissolved in the dilute sulphuric acid, and the quantity of gas disengaged during the solution accurately ascertained. The plumbago which remains after solution is also separated by filtration, and carefully weighed. Now it is well known that the purer the iron, the greater the quantify of inflammable gas obtained, and the less the proportion of plumbago which remains after the solution; from these two parts therefore a tolerable judgment may be formed of the quality of the metal. When the plumbago exceeds 4½ _per_ cent. the iron will always be found deficient in strength; and there has been no instance of a gun bursting where the plumbago did not exceed 3 _per_ cent; that is, where 100 grains of the metal did not leave more than 3 grains of plumbago. The colour of the plumbago is also to be attended to; when it is brown or reddish, it is an indication of hard metal, and when in quantities and mixed with coals, there can be no doubt but that the iron is too soft for cannon. For the proof of _Gunpowder_, see that word.