The cases for wheels must be as strong as those for sky rockets, as they have to bear a considerable force, in consequence of the strength of their charges. Nor is the weight any objection : on the contrary, when once it is in motion, a heavy wheel acts better than a light one, because it does not easily lose the velocity which it has acquired ; but, acting like a fly wheel, maintains during the burning of any slow part of the composition which may be introduced, that motion which it had acquired from a stronger one. These cases need not, however, be limited in length as those of rockets are, but may be made of twelve diameters, or even more. But a foot of length will be found a very good general rule for a case whose interior diameter is an inch.
When driven, a little clay may be laid on the mouth, if the artist thinks proper, to preserve the choke from being burnt ; and if this is done, and the cases are sufficiently strong, they will admit of being used a se cond time. The ends are to be left open, and no bounces are to be admitted, as they would be apt to derange the wheel, and throw it off the spindle. If a bounce is wanted at the end, which may be desirable, it can be attached loosely like a maroon to the last case by means of a leader, and laid upon one of the spikes, so as to disengage itself when it takes fire, without injuring the machinery. The same may indeed be done more ex tensively, as we shall hereafter notice in treating of hexagonal wheels.
In filling wheel cases, three or more compositions may be used, and for the nature of these we may refer to the table. But we will here suppose that three have been selected, namely, an iron sparkling fire, a common charcoal one, and a blue or white flame. A diameter of the case or more, near the mouth, must he laid with a strong charcoal composition, and that is to be followed by the same quantity or more of a brilliant one, which may again be succeeded by a blue or white flame, and so on alternately, as the taste of the operator may direct. It must be remembered, however, that the flaming compositions should never be placed far away from the mouth in the cases, as these flames will not reach very far, and might be suffocated or burnt out within the rocket, instead of showing as they ought to do. The artist must also recollect that half a diameter of any of these will be sufficient, as they burn comparatively slow ; and as their force is very small, they might, if they lasted too long, retard the motion of the wheel inconveniently. Whenever there is any
fear of this event happening, a strong one must be made to succeed, for the purpose of recovering the ve locity.
The artist must now, however, recollect, that if a case is to be bored, no such changes of the composition are admissible ; as the bore would render all this useless, by causing the different kinds to burn together. In such a case, any one composition must reach at least to half a diameter beyond the bore, after which a change is admissible.
The last general direction applicable to all wheels alike, is the fixing and priming. Supposing the case of an hexagonal wheel, as applicable to all, the rockets must be tied on the circumference with strong twine, which is afterwards to be covered with a turn or two of pasted paper. Great care must be taken that they are placed in the plane of the wheel, so that no force may be lost and that they are placed in succession so that the choke of one is next to the end of the preced ing. A whole diameter of the case must also be left between each pair, that the fire may flow freely with out meeting any obstruction from the end of that last burnt. This being done, a thick strand of quick-match must be introduced between each head and tail, and fixed in both; when the joint must be secured with pasted paper. This should not, however, be made too strong, to prevent the wheel from being shaken by the explo sion which takes place at the lighting of the cases. Lastly. a long leader is to be fixed to the first, and being reserved closed, it may afterwards be cut to the proper length, and affixed to any firework which it is intended that the wheel should follow.
Of Single Case Wheels.
These are extremely convenient for attaching to other fireworks, where there is little room to spare, or where much weight cannot be carried ; and, from their acting at two orifices at once, the effect is very good. They are, however, of very short duration, so that they are not fit to be used alone. In the Chinese drum they are indispensable ; as the necessity of packing close renders any other kind of wheel scarcely admissible. They may also be attached to line rockets, and to many other classes of fixed and moving fireworks; but we need not give particular directions for these uses, as a dexterous artist will easily learn to avail himself of the advantages which they offer.