Of Compound TVheels.
There are various methods of compounding wheels and they are all so easy in the execution, and so bril. liant in the effects, that they well deserve the attention of pyrotechnists. They are, however, almost solely limited to vertical movements; as in the horizontal ones the effects are lost to the eye. We shall describe a few of the most striking, aiding the descriptions try proper figures, which will be found in the plates, as tc describe the whole is unnecessary, since they admit o: endless variety.
Concentric, Direct, and Reverse Ineets.
There are two modes of performing this, and they also admit of some subordinate varieties. Two sets o cases may be applied on two circles, one within the other on the same spokes. In such a contrivance, bott wheels may act at once, and for the whole time, or one may commence when the other is partly exploded. The artist will, of course, easily contrive to regulate the time; the great object being that they may both end together, however they may begin. The outer wheel (Fig. 27,) for this purpose ought to be of consider able size, that the fires may not be too close. The inner may also differ in composition from the outer. Thus, if it carries a blue light, while the other is a brilliant com position, there will be two circles of 'different fires ; and as the blue fire is far slower than the sparkling one, they may both be managed to burn together from the beginning to the end. Otherwise it may be so managed by laying a leader from any part of the outer circle to the inner one, that this latter shall not com mence till some time after the first. It is equally plain that where there is room enough, more circles than two may be adopted, while the fires of all the three or four, and the times of their commencement may also be made to vary. Thus, to put an example, suppose an outer circle of twelve cases, and three inner ones in succession, of nine, six, and three cases, of the same length and of the same composition. A leader is to be conducted from the third case of the outer circle to the beginning of the second ; another from the third case of that to the beginning of the third circle; and the last from the third case of this one to the beginning of the last circle. Thus, the wheel which commences
with one circle of fire, will become double, and at length triple, and quadruple, all in concentric order, and all terminating at the same time. A light may also be attached outside the spindle of such a wheel, which may be properly fitted to receive one without difficulty ; thus forming a bright spot on the centre of all these fires.
But concentric wheels may also be made to produce a different effect by giving a different velocity to the two or more circles. This is done by having two dis tinct wheels of different sizes, on one spindle, working independently of each other. By means of a stronger composition, or by boring, the inner wheel may be made to revolve more rapidly than the outer one, to which, from its size, it has also a natural tendency. The same may be done for more wheels, it being only necessary that each should be separated from the other by proper buttons, placed in the spindles between them. If small wheels only are used, this effect is easily produced in a very simple manner, by attaching a common pinwheel on the top of the spindle of the larger one.
It is by this last method that concentric reversed wheels are produced ; and the effect of these is even more remarkable than that of the former. It is unne cessary to give directions for making such wheels, as it must be sufficiently obvious ; it being only requir ed that the mouths of the cases may look in oppo site directions. But a large kind, of which the effect is very peculiar, may be made by placing four cases on as many long arms, without any wheel ; connecting them by proper leaders. Two of these, of the same size, being placed in reverse order, one before the other in the same spindle, when they arc fired they do not produce circles, from their great length and slowness of motion. Instead of this, two curved lines of fire appear to meet and separate alternately, something like a pair of forceps, causing a very singular appear ance, which the spectators, unaware of the contrivance, arc puzzled to account for. The effects of all other opposing motions, whether in large or small circles more or less numerous and concentric, are easily under stood.