The pig-iron melted by the flame playing on it, flows down into a cavity L, which has a hole T opening outwardly, and stopped with clay. When the hole is forced open by a workman, the metal is sues and is oonveyed by a gutter formed of sand to the gun-mould, into which the melted metal falli through the open end of the head. The sand form-. ing the gutter should be in a proper state of mois ture. If it is too dry, some pieces of it will be car ried away by the metal. Across the gutter is a dam composed of an iron-plate luted, and dipping a lit tle below the surface of the metal to retain the sco rite. This dani is driven down to stop the current of metal when the mould is full. The metal is also skimmed, as it passes along, by a skimmer, composed of a rod Of iron 'terminated by a flat semi-elliptical piece luted and made red hot. It is sometimes the practice to plunge a piece of green wood for a time into the head whilst liquid, This is with a view to prevent honeycombs, and its action may be to rnetallize any oxidated particles of the metal ; and that the vapour from the green wood rising to the surface of the metal may carry with it small air bub bles, or other extraneous bodies that would, if they remained, occasion cavities in the metal.
The figure is a transverse section of the air-fur nace. C is the opening through' which the coals are introduced. P the opening at which the pig-iron is thrown in. T•the hole through which the metal. is let out. The metal flows into the casting-house. 0 the floor of the casting house. In this floor is the pit in which the moulds of large goods are sunk, that the metal may flow down into them. I the door, with a hole in it, for seeing the state of the melted metal.
G the grate. L the lower part of the cavity of the furnace, into which the metal, as it is melted, flows. S steps leading to A, the ash-pit., N bottom of the furnace lined with sand. H chimney; the height of the stack is 40 feet from the surface of the ground. The stack is strengthened in different places by iron bars, X. F is the mass of building which forms the foundation built below the surface of the ground to support the weight of the furnace and stack. .R. the surface of the ground out of doors. CPNL H is the course that the flame takes. • • .
It is better to cast the guns from the air-furnace than from the blast-furnace ; for in the blast-furnace, where the ironstone is smelted, the quality of the metal is uncertain, and it may vary from one cast to another, by causes either unknown, or not under the control of the iron-master. On the other hand, in the air-furnace, pig-iron of a quality proper for.mak ing guns is put in, and the quality of the iron is not materially altered by the process of melting. . • The head of the gun is like the jet (gate or Beet of the workmen) of any other casting. Whilst the whole is liquid, the head is a column of liquid metal that acts by its height, exerting pressure on the me tal that forms the body of the gun. The metal sub jected to this pressure whilst liquid, is less subject to porosity when cooled. The head also furnishes me
tal to fill up the cavities that occur, in the piece by, the contraction and crystallization of • the metal whilst it is passing to the solid state. All the great contractions and crystallizations are thus transferred to the surface of the head, which is found, to' 134 composed of cavities, sometimes containing cast-iron crystallized in a fern-leafed shape. The head tin the beltmetal was that retlieed to the state of a coarse powder; this powder was thrown into ano ther qua ntity of bait-metal in fission ; the metallic and oxidatedper in the powder melted, and wu mixed with the festal bell-metal; the oxide of tin of the powder remained on the surface. A melted mass was dine obtained, containing a larger proportion of copper than the bell-metal, and fit for making guns.
The mould for brass gums is formed of dry sand, in the same way as the mould for cast-iron guns al ready described. The furnace for melting brass for guns is a reverberatory furnace, the metal being ex posed to the flame. It has no high chimney like the air furnace for melting iron, the heat required not being so great as that for melting cast-iron.
Brass guns are subject to melt at the interior ex. tremity of the touch-hole, by the heat of quick fir. ing ; and the melted parts are driven out by the ex. plosion, se as to render the touch-hole too wide. To prevent this, there is sometimes a bush of cep. per inserted, and in this busti the touch-hole is dill. led. The copper being less fusible than the brass, is not melted by the heat of firing the piece. To form the bush, a cylindrical piece of copper is ham mered cold, and made into the form of a male screw, A hole is then bored, reaching from the surface of the gun into its bore ; the diameter of this cyan &kid hole is equal to the diameter of the cylin der of copper measured from the bottom of the threads of the screw. The piece of copper is then screwed into the cylindrical hole, and touch-hole is drilled in it.
The improvement* in casting cannon, as in other arts, have been gradual. Formerly cannon were cut with a core, and this was practised, in some founderies in Flanders, even in the year 1794. But they are now always cast solid, experience having shown that guns cast solid are stronger, and less liable to bunt, and the bore is freer from honeycombs, and more likely to have the same axis with 'the piece. The second of these qualities is still more certainly at tained by the practice now in use, of making the gun itself revolve whilst boring ; in this way, as long as the boring bar remains unmoved, the axis is right ; but if the boring bar has a conical motion, then the point of the bit is out of the axis; when the boring bar was made to revolve, the bore might deviate greatly from the axis. The improvements in the casting of cannon have kept pace with the in. provements in the manufacture of cast-iron.