The quality of pig-iron is known by the appear ance of its surface, but more decisively by the ap pearance which its fracture presents. To obtain this fracture, a man takes one end of a pig in each hand, and liking it as high above his head as be cm, throws it with force, so that the middle of the pig shall fall across another pig placed on the ground ; in this way the pig thrown down is broken. Soft or grey pig-iron, which is the most valuable, breaks with difficulty, and the surface of its fracture is of a grey colour, composed of pretty large crystalline grams. Hard or white breaks easily ; the surface of the fracture 18 white, and not sensibly granulated, the grains that compose it being small. The pig iron here spoken of is that smelted by the • conk ofpit-mal. Pigaron smelted with charcoal of wood has a fracture of a different appearance, some times lamellar, like the fracture of a metallic bis muth. Formerly guns used to be cast from the blast-fartiace ; that is to say, immediately from the ironstone. This was attended with uncertainty is respect to the nature of the metal ; for the nature of the metal given by the blast-furnace varies- frequent ly and suddenly, from causes either unknown, or not .under the command of the iron-master. For this reason guns are no longer cast from the blast-fur nace, but pig-iron already formed is taken, of such qualities, and in such proportions, as to form a metal neither too soft nor too brittle and hard for guns. The different kinds of pig-iron thus selected are melted together in a furnace called, in iron manufac tories, an air furnace, and by some writers a rever beratory furnace, by the flame of pit-coal; the flame being impelled by a strong current of air produced by the rarefaction of the air in a chimney of thirty or forty feet in height, the column of the atmosphere of which the air in the chimney makes a part, being lighter than the unrarified columns of the atmo sphere next it, its equilibrium with these columns is destroyed ; the neighbouring columns therefore rush through the grate of the furnace, which is the only aperture by which they can attain the bottom of the rarifled column, and they carry the flame of the coal against the pig-iron, which is thereby brought into fusion. From the iron thus fused only one large gun is cast at one time, the furnace not being capable of melting more metal than is regal. site for that purpose.
The gun is cast with two appendages, which are to come off before it is finished and ready for use: The one is a square piece beyond the cascabel, for fixing the gun so as to revolve with the axis of the boring-mill ; the other is the bead.
The heal in cast-iron cannon is a mass of cast itbn two or three feet long, somewhat bell-shaped. It is a prolongation of the mass of metal beyond the muzzle ring, and in the position in which the gun is cost, the bead is the top of the whole mass, the square beyond the cascabel being the lowest part. After the metal has cooled, the upper surface of the head is cavernous, as is the case with the surface that is uppermost during the cast ing and cooling of any body of cast iron: the sides of the cavities in the head are frequently formed of cast iron crystallized in a fern-leaf form. The intention of the head is to prevent these cavi ties, which are formed most abundantly at the upper surface of the cooling cast iron, from forming in the gun itself. But, notwithstanding the precaution of casting the gun with a large head, and of mixing proper kinds of cast iron in the air-furnace, it fre quently happens that small cavities occur in the guns.
The gun with its head being cast and allowed to' cool, is taken to the boring-mill, where the head to be taken off, the cylindrical cavity or bore is to be formed, and the outside of the gun is to be turn ed. Formerly the boring of guns was done in an
upright position ; the gun being placed above the boring-bar was fixed in a frame sliding vertically in grooves ; this frame was suspended on each side by a block and tackle, and the end of each of the two ropes was wound round a windlass. By turning these windlasses, the gun might be raised or lowered, and by this means might be allowed either to press with its whole weight on the boring-bit, or with any part of its whole weight. A figure of this 'apparatus may be seen in the French Excyclopeldie Planches--Fonte. Another vertical apparatus for boring cannon is represented in Rinman, Bergwerks Lexicon, Stockholm, 1789, Tab. IV.
The practice which has long been followed in this country, is to place the gun horizontally-in the boring ; and it is fixed on the axis of the mill by means of the square piece at the cascabel.
In a boring-mill constructed by Smeaton, one gild is placed on the horizontal axis of the water-wheel itself, and, consequently, revolves with the same ve locity. On this same axis is a toothed wheel with 78 teeth, which works two wheels, one placed on each side of it, and each having 29 teeth ; on the axis of each of these a gun is placed ; their power is lilth of the power of the centre wheel. (See Smeaton's Re ports, Vol. I.) On the axis where the power is least, smaller sized guns are bored ; on the axis of the greatest power, the large guns are bored. A crane moveable on a vertical axis, with sweep that extends over all the carriages, with a tackle hanging from its beam, and wrought by a windlass, serves to place the gun on the carriage where it is to be bored, or to remove it from one carriage to another if required ; and afterwards, when the gun is bored and turned, the crane serves to remove the gun from the boring-mill.
The gun, when placed on the machine, has the square at the cascabel fixed in a square iron box (G, Plate XXXVI. fig. 5.), on the axis. This box has a screw passing through each of its sides, and by the operation of these screws, the square of the gun is adjusted, centered, and fixed; the chace of the gun is also fixed in a collar N, in which it is to revolve. (The collar in the figure is represented too near the muzzle ring.) The axis on which each gun is fixed, may be set in gear or put in connection with the revolving axis of the machine, so as to move round with it, or taken Out of gear, so'as to remain at rest, although the other parts of the machine continue in movement. There are various methods of doing this; one is given by Smeaton in the work above cited. Alter the gun is fixed on the axis, and before beginning the opera tion of boring, the head, which has been described above, is cut off near the muzzle ring : for this pur pose, the gun is set in gear so as to revolve on its axis with the moving power ; and a bar of steel, in shape and size like the coulter of a plough, is ap plied at right angles to the axis of the gun ; the nar row side of this bar is sharpened to a cutting edge, so that it has the form of one tooth of a very large saw ; and this cutting edge is opposed to the direc tion of the redolving motion of the gun, and held strongly on to the gun by a screw pressing on the bar; the cutter takes of an angular portion at right angles to the axis, till the cylindricalconnect ing the head with the gun is so that the head is made to fall of by the blow of a hammer applied on it. In brass guns, cast with a core, the head was sawed off by hand with a blade of steel, whose edge was toothed as a saw, and the sides toothed as files. See the French Encyclopidie rlanchesFolde.