The rockets for this purpose must be disposed round a central tube, well fitted to the line, and very free to move. The tube carries, by means of spokes fixed in it, a wooden wheel of the usual construction, round which the rockets forming the wheel of fire are to be fixed Thus the tube forms the centre of the wheel, and the line its spindle. It is plain that if this is well managed, the wheel will continue to revolve while the rockets retreat, and the same will take place as it re turns. If the wheel is to be small, a single rocket at a time will suffice to carry it forwards ; if large and weighty, it will be preferable to fire two rockets at once in the same direction, and to provide two in the same manner for the return ; and these matters are, of course, easily managed by a proper disposition of the leaders. In constructing this compound firework, the two parts, or the wheel and the line rockets, must be timed, so that the whole may expire together. Thus, for example, an hexagonal wheel of six cases would burn with six line rockets separately. If there are but four of these, the wheel may be a square, as the form is of no moment ; or else two cases on opposite sides of it may be fired at once, which will add much to the brilliancy_ of the Orert mh. °HUM, al ansetrient must oe made it two of the line cases are arranged to burn together. We need only add, that in firing this rocket it is better that the wheel should be fired at first, and perhaps even suffered to burn for a considerable time before the line rocket is kindled. It will thus appear to the spectators as a common fixed wheel, and the surprise will consequently be the greater when it is seen unexpectedly to fly away. It is a good rule on all occasions for pyrotechnists to recollect, that to ex cite surprise by something that was unexpected, is an essential part of their business, and that their inten tions should, therefore, be concealed as much as pos sible.
There is yet another way in which a line rocket may be caused to produce somewhat of the effect of a wheel, but it must be double, or two must burn toge ther. For this purpose the tube which is to receive them, must be so made as to receive them in an oblique position instead of a parallel one, and in such an order as to form the two consecutive parts of a spiral. Thus, when fired, the tendency of each to recoil will be com bined with a spiral one, so that in retreating they will revolve round the line. It is evident that the balance must in this case be made perfect, as the accurate per formance of the firework will depend principally on this. By altering the angles which the fireworks make with the axis of motion, the circle of fire may be varied accordingly ; but it must be that as the circle is enlarged, the velocity of direct motion will be diminished, and the reverse.
This variety of the line rocket may also be doubled. To effect this, four cases will be required, two for the retreat, and two for the advance or return ; and they must, of course, be placed parallel, so that the same direction of the revolution may be maintained in one way as in the other. It is easy to understand how the
leaders are to be managed in this case ; while it must be recollected that to preserve the balance, the opposed 'eases must burn together.
Of Tourbillons.
This is infinitely the most brilliant and surprising of all the simple fireworks; but, unfortunately, its career is as short as it is splendid, as there is scarcely time to see it before it is burnt out. These fireworks should, therefore, be numerous in all public displays ; though, from the difficulty of making them, artists are very apt to be economical in their use. Yet, as these are cheap in proportion to their brilliancy, and cannot fail to suc ceed in good hands, they ought never to be omitted. We have given a drawing of the construction of this firework, Plate CCCCLXXII. Fig. 8. as it is difficult to make it by a verbal direction only. The principle is simple, and, by explaining that first, the reader will the better understand the following directions. Its effect is that of a wheel which revolves and ascends at the same time ; and, as this is done by a single case, in a most ingenious manner, it is evident that it requires two dis tinct kinds of recoil.
The one pound case is the largest that should be used for the tourbillon, and it is of very little use to make them less than the half of that size, or eight ounces. The length may vary from eight to nine di ameters. Before driving, the case should be choked close at the lower end, and it must be driven as hard and as carefully as a sky rocket. Ordinary rocket composition may be used for it, or, what is better, a strong brilliant fire with iron, a receipt for which will be found in the table of compositions. When the case is nearly filled, the tipper end of it must be turned in with a little glue, fold after fold in succession, and well beat down with the rammer, so that both ends may be alike secured.
The case thus ready, must be bored either with a greased giniblet, or with the drill, as formerly describ ed. As every thing in the performance depends on the boring, this requires great attention. A hole is first to be made close to each end of the case, but on oppo site sides, and equal in diameter to half the diameter of the charge, or exactly equivalent to the bore of a sky rocket. By means of these holes, the revolution of this firework is effected, so that it is so far a single case wheel. On that side of the case, which is a qua drant removed from these two holes, or which would be the bottom side of it were it laid on a table with a hole looking sidewise each way, a line must then be made, and divided into three equal parts between the space which is included between the two holes just de scribed. A hole must now be bored in each of the two middle points just described, at right angles to the axis of the rocket, and to the direction of the side holes also. Thus the composition is divided by means of these two holes, and the first two at the ends, into three equal parts. The last holes must he of the sante diame ter as the first, or equal to one-half the diameter of the coin position.