Page:The American Cyclopædia (1879) Volume III.djvu/721

 CANNON 711 the muzzle part up, carefully surrounded by the corresponding part of the jacket, and when properly adjusted the sand is rammed about it. The model for the body of the piece is then placed on top of this, and the corresponding parts of the jacket adjusted and filled as before. The patterns for the trunnions, rimbases, and cascable are affixed and filled in a similar man- ner, and the mould completed. Care is taken to cover each portion of the model with coke wash, and to sprinkle dry sand upon the top of each section of the mould, to prevent adhe- sion when they are separated. In the body of the sand within the flask a channel is formed, in the same manner as the cavity of the mould, for the purpose of permitting the molten metal to be introduced at the bottom of the mould, so as to prevent its injury by falling metal. Jf any portion of the mould is injured by with- drawing the pattern, it is repaired, and the in- terior of the mould then covered with coke, wash, after which the several parts are placed in an oven and gradually dried. When this is finished, the parts are carried to a pit, reunited, and secured in a vertical position, breech down- ward. Pains are then taken to see that all parts of the mould are perfect and properly adjusted ; the melted metal is permitted to flow in at the bottom of the channel made for that purpose, and as it rises in the mould a work- man agitates it with a long wooden pole, taking care that the scoria and other impurities are prevented from getting into the cavities for the trunnions. After the mould is filled, it is per- mitted to stand from three to twenty days imbedded in sand, so as to cool gradually. When nearly cold the gun is hoisted from the pit, taken out of the flask, and cleaned of the sand. All solid bodies contract in cooling. If they cool unequally they will also contract unequally, and thus undergo a change of form, unless restrained by the presence of a superior force. The general form of a gun casting is that of a solid frustum of a cone; and in cooling from the exterior, the thin outer layer contracts first, and forces the hotter and more yielding metal within toward the centre of the mould. The next layer cools, and tends to contract; but the exterior to which it coheres has become nearly rigid, and does not fully yield to the contraction of the inner layer. The result is that the cohesion of the particles of the inner layer is diminished by a force of extension, while that of the outer layer is in- creased by a force of compression. As the cooling continues, this operation is repeated till the whole mass is brought to a uniform temperature. All cannon, therefore, that are cooled from the exterior, are subjected to two straining forces : the outer portions to a strain of compression, and the inner portions to a strain of extension; and these forces exert themselves upon the different parts of the gun with intensities proportional to their distance from the neutral axis, which is composed of particles so situated as to be neither extended nor compressed by the cooling process. The ef- fect of these strains may be so great as to crack the interior metal of cast-iron cannon while they are cooling. These considerations led Capt. (afterward Gen.) Rodman to invent the plan, now generally adopted, of cooling cannon from the interior, by passing a stream of water through a hollow core inserted in the centre of the mould cavity before casting, and sur- rounding the flask with a mass of burning coals to prevent the radiation of heat from the exterior. By the use of Rodman's method cast- iron guns of 20-inch calibre have been made, and found by experiment to withstand the vicissitudes of usage admirably. After the piece is cooled, it is fastened upon a rack and lathe, and the sprue head cut oiF. If cast solid, the boring is commenced by means of a piercer, which bores a small hole along the axis to the bottom of the chamber; this is followed by a second cutter called a reamer, which completes the bore to the chamber. The reamer is then removed, and its place supplied by the chamber cutter, which gives the proper form to that part of the bore. In pieces cast hollow, the piercer is not used. While the boring is taking place the outside of the piece is turned to the proper shape, except at the trunnions, which are turned down by another machine. The vent is bored while the gun is in the trunnion lathe. The time required to finish a cannon depends upon its size ; a 10-inch columbiad requires six weeks, while a 20- inch Rodman requires from 12 to 18 weeks. The manufacture of wrought-iron or steel cannon is much more complex and attended by much greater difficulties than that of cast- iron or bronze. This is due to the fact that no machinery has yet been devised for heat- ing, handling, and forging masses of wrought iron or steel sufficiently large to construct the largest cannon without welding; and this, even when most skilfully done, is always more or less imperfect, and renders the piece liable to explosion. The term " built up " is fre- quently applied to all kinds of wrought can- non, in which the parts are formed separately and then united together. The object of this mode of manufacture is to secure guns of ma- terials more elastic and durable than cast iron or bronze, or to correct the defects of one material by combining it with another of op- posite quality. Among the earliest applica- tions of this idea was the trial made to in- crease the hardness and therefore the endu- rance of bronze cannon by casting them around a core of steel which formed the surface of the bore. But built-up cannon are not necessari- ly composed of more than one kind of metal ; some of the most noted are made of steel or wrought iron alone. In these cases the de- fects which usually accompany the working of large masses, such as crystalline structure, cracks, and false welds, are partially avoided by forming them in small masses, as rings and tubes of good quality and excellent workman-