The brass mortars and howitzers are fired twice with their chambers full of powder, and an iron shell. The mortars on their own beds, at about 75 degrees elevation; and the howitzers on their carriages, at about 12 degrees. Iron mortars are proved on their iron beds, with a charge equal to the full chamber, and an iron shot equal in diameter to the shell.
Cohorn 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 a 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 diluted 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 quantity 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 4¹⁄₂ 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 color 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.
Proof of Iron Shells. After the shells are guaged and examined as to their dimensions and weight, they must be well scraped out, and the iron pin at the bottom of the inside must be driven down or broken off. They are then to be hammered all over, to knock off the scales, and discover flaws, and no hole, in the large shells is allowed, of more than ³⁄₄ of an inch deep. An empty fuze is then driven into the fuze hole, and the shell is suspended in a tub of water, in such manner that the shell be covered by the water, but that it does not run into the fuze: in this situation the nose of a pair of bellows is put in at the fuze hole, and several strong puffs given with the bellows; and if no bubbles rise in the water, the shell is concluded to be serviceable.
Ordnance condemned as unserviceable for any of the foregoing reasons, are marked as follows: ╳ D, or ╳ S, or ╳ W. The first signifies that they are found to be faulty in their dimensions, by Desagulier’s instrument; the second, by the searcher; and the third, by the water proof.
Proof of powder, is in order to try its goodness and strength. There have been different inventions proposed and put in practice heretofore, for the proof of powder. See [Gunpowder], and [Eprouvette].
Proof of cannon, is made to ascertain their being well cast, their having no cavities in their metal, and, in a word, their being fit to resist the effort of their charge of powder. In making this proof, the piece is laid upon the ground, supported only by a piece of wood in the middle, of about five or six inches thick, to raise the muzzle a little; and then the piece is fired against a solid butt of earth.