It may be made of any form or number of cases, but it is necessary that both the wheels should be attached to one frame, and work on a common centre, as the leaders could not otherwise be made to communicate. Both the wheels should also be of the same duration. This may be managed either by making the composi tion of the inner one twice as slow as that of the outer, or else by doubling the same cases in the inner. Thus, if the outer wheel consists of six cases in hexagon or der, the inner may have the same number disposed in a double triangle. In uniting the leaders, care must be taken that the tendencies of both wheels are pre served in the same direction; and that we may render the disposition of these more intelligible, we have chosen a simple form on which to represent it.
Of Combinations of Fixed Cases.
The forms into which these may be considered are almost endless, and there is indeed scarcely any limit to them but the expense. We shall only here describe a few of the most practicable, as from these the artist may easily extend them to any number he thinks pro per by variously combining them. We have also given figures of these, which will save a good many words in the description.
The general rules are alike applicable to all. For sonic, such as all the fires which contain iron, the cases must be choked, as they must be considered as gerbes; and these compositions must be varied with iron-fire, and with common charcoal fire. They must also be provided of two or three different lengths, and also of different sizes, for the sake of varying the effects. Be sides these sparkling cases, there must be ready a num ber of cases, of different sizes also, of white and blue lights, together with crackers, stars, serpents, and Ro man candles. With these at hand, the artist will be able, in a very short time, to make up all the different forms that he may fancy, instead of being checked in his operations for want of materials. We need scarce ly say that various shaped frames will also be required ; but as all these arc immoveable, there is no difficulty in their construction.
These combinations are generally made out of large cases, seldom being formed out of such as are less than an inch in diameter; as, when upon a small scale, their effects are not sufficiently distinct. Such cases are also often required to be of an unusual length ; but as it is not convenient to drive them longer than eighteen inches, if more is wanted, it is better to join two to gether in the same manner as the cases of pin-wheels are united. But as such very long cases will not burn
out well when the orifice becomes too distant from the fire, the paper case must be made so thin that it will burn down ; or, what is better, a part of it may be un rolled after driving. It is chiefly where a white light is wanted to burn for a great length of time, that this precaution is required.
It seldom or never happens that these combinations are used alone. They are sometimes the termination of some other simpler display, or else they form part of some general system of mutations. In all cases, they must be provided with a general leader for the purpose of setting them on fire, independently of the particular ones which, by firing all the cases in their proper or der, are to produce the intended figure in fire. If they are to follow after wheels, as these cannot communi cate light to any thing, that leader must be in the power of the operator ; otherwise it is fastened to the end of the firework, of whatever nature, which it is in tended to follow. But the greatest care must be taken in the adaptation and protection of these, that they may not fail to communicate, and that at the proper time, they may not take fire accidentally from the sparks which may be flying about, and that, when they do fire, they may not burst in such a'manner as to derange themselves, or any thing about them.
Geometrical Figures.
The chief of these are crosses, triangles, squares, hexagons, and octagons ; beyond which the figures ap proach too near the circle to produce a distinct effect. (Plate CCCCLXXIII. Fig. I In the cross, the four cases are disposed with their mouths outwards; and the effect of this firework is much improved, by placing a bright light or a small wheel in the centre. Or else four lights may be at tached upon the cases, so that the same ease may ap pear to be throwing out both the sparks and the light. In many fireworks, this is a deception which may be used with very good effect.
The triangle, square, &c. all act on the same prin ciple, so that the same rules will serve for the whole. 'Wherever there is an angle, two cases meet at that point, so that their fires cross outwardly, these produc ing a variety of regular and pleasing figures. Thus in the square, for example, they form four external right angles; in the hexagon, six triangles.