All these, except the smaller mortars, are made of cast iron. This substance is less tenacious than wrought iron or bronze, and the cannon made of it are, on this account, much heavier than of the other materials; but for the naval service, and the attack and defence of fortifications, the weight required to secure the necessary strength is not very objectionable. Wrought iron and bronze are much more expensive and less durable. Moreover, the difficulty of forging wrought iron in masses of sufficient size has been such as to prevent its being brought into general use for artillery. Numerous attempts have been made, at different periods, to construct large guns of this material, but none have yet been successful. Improvements which are now making in the manufacture of wrought iron, may render this the preferable material for the smaller pieces of artillery; but the best informed military men deem it objectionable for the heavier cannon, both on account of its cost and the imperfection of its manufacture. Even should the latter objection be removed, its cost must prevent its general application to the construction of siege artillery. Charlatans in military science, both in this country and in Europe, bring this subject up every fifteen or twenty years as a new _invention_, and flaming notices of the _improvement_, and predictions of the revolution it is to effect in the art of war, are circulated in the newspapers to "gull" a credulous public; and after some fifty or one hundred thousand dollars have been squandered on some court-favorite, the whole matter ends in the explosion of the "_improvement_," and probably the destruction of the "_inventor_," and perhaps also of his spectators. Let us be distinctly understood on this subject. There may be _inventions_ and _improvements_ in the manufacture of wrought iron, but there is nothing _new_ in its _application_ to the construction of cannon, for it has been used for this purpose as long ago as the first invention of the art. 2. _Field Artillery_ is composed of the smaller guns and howitzers. In our service this class of cannon includes the six and twelve-pounder guns, and the twelve and twenty-four-pounder howitzers. All these are now made of bronze. This material is more expensive than cast-iron, but its superior tenacity renders it more useful where great weight is objectionable. Improvements in the manufacture of cast iron may render it safe to employ this metal in the construction of field-pieces. It is also possible the wrought iron may be forged in masses large enough, and the cost be so reduced as to bring it into use for field-pieces. It is here important to combine strength with lightness, and additional expense may very properly be incurred to secure this important object. The _projectiles_ now in use are solid shot, shells, strap-shot, case or canister-shot, grape-shot, light and fire-balls, carcasses, grenades, and rockets.