On the Moulds for Fireworks.
The moulds of fireworks, see Plate CCCCLXXII. Fig. 1., are the tubes in which the cases are to be inclosed during the time of filling, to prevent them either from bursting or sinking downwards under the rammer during the time of driving. They are absolutely indispensable in all works that 'require driving, and can indeed only be dispensed with in spiral wheels. The general principles of the construction are the same for all ; but as the rocket mould is the most complicated, we shall first describe that one; the others will be easily understood.
The rocket mould, if on a small scale, may be made most conveniently of gun metal; if large, of stout wood, such as oak, elm, or, what is best of all, lignum The upper part is a stout c ylinder, generally turned, with some ornamental moulding both above and below, so as to resemble a short column in architecture. If in wood, it must be hooped at each end with a ring of metal, properly secured by means of pins. The bore of this cylinder passes through it entirely, and is of the exact diameter of the case which is to be driven. The pro portions of all these parts are regulated by those of the rockets; but we may here observe, that the length of this cylinder, or of the bore which receives the case, is about seven diameters. It may be more without incon venience, but cannot well be made less. If, on putting the case to it, that should prove too tight, part of the external paper is easily cut off; if otherwise, a little more paper should be added, as it is essential that the case should fill the mould perfectly.
The lower part of the mould consists of a cylinder of the same materials, but of nearly double the diameter, or even more, that it may be enabled to stand firmly on the block on which the work is driven. It is even better, particularly in large works, that a cavity should be made in the block to receive it, as we remarked some time ago. A large cavity in which the upper cylinder may be fitted, and about an inch or more in depth, is made in it, so that, when applied, they may both form a solid piece, as it were, taking care that the meeting is quite perfect below, and that the upper cylinder stands perpendicular and firm.
It is usual now to have two of these bottoms to each upper cylinder, one of them bearing both a nipple a, and a spindle b, and the other a nipple a, alone. The latter
is required for receiving the rocket when the clay wad ding is driven down ; and it is also convenient for fill ing the class of cases that do not require boring. The spindle is of course required for all works which, as well as rockets, arc to have a cavity. But one bottom may serve for both purposes without difficulty. For this end, a hole is to be bored in the lower cylinder, yet not quite through it, exactly corresponding with that one in the upper piece of the mould. To this must be fitted two metallic cylinders, one with a hemi spherical nipple turned on the end, the diameter of is less than that of the whole bore, and equal to that of the case. The other carries the spindle, which may either be turned in one piece with it, or fitted into the nipple, and then firmly secured. It is so essential that the spindle should be precisely in the line of the rocket's axis, that too much care cannot be taken in making it true and firm, that it may be liable to no accidents.
The diameter of the spindle at the base, as we already noticed for the bores of rockets, is half the interior diameter of the case, and is to be fitted exactly to the choke or vent. This is done, by using a wire of the same dimensions in the subsidiary piece of the former, which we ought to have noticed before, and which is inserted into a hole in it, to be used when the case is choked. The length for rockets is within a diameter and a half, or nearly, of that of the composi tion; but it is more particularly detailed in our tables of rockets. For wheels the diameters the spindles are the same, but the length need not exceed one or two diameters of the composition, according to the initial velocity which the artist wishes to communicate. Lastly, to keep all these parts firm and steady, a hole must be bored horizontally through the lower cylinder and upper, including the piece which carries the nipple. To this a wire a b, furnished with a ring, a, for withdrawing it, is fitted, which being passed through, and secured on the further side with a forelock if necessary, the mould is ready for use.