It may now be understood, that if this firework were laid with these two intermediate holes directed down. wards, the fire issuing from them would give it a ten dency to ascend, by means of this vertical recoil. But, although this is the case to a certain degree, there is yet not power enough for that purpose. To obtain this, a hole must now be bored, as nearly as possible in the direction of the axis of the rocket, from each of the two middle ones towards the end, and until they have reached to within a diameter of the composition of each end hole. Thus the tourbillon becomes a kind of double sky rocket, as far as the two lower holes are con cerned, while it continues to be a wheel by means of the end ones. It is evident, therefore, that it will now ascend by the recoil from the lower holes, while it will revolve on its centre by that from the end ones, and the velocity will increase as all these holes come, in the pro gress of burning, to communicate together.
To complete this rocket for firing, all the holes must be caused to communicate by a single leader. A stick is then to be tawened on it transversely. This is made of a thin and light piece of deal, and the fastening is made so that it lies een the two bottom holes, and in such a manner as to point on which it may revolve freely when placed on a table. Care must, at the same time, be taken that it is perfectly and nicely balanced on this point, without which its performance will not only be incorrect, but very dangerous to the bystanders, should it, in flying off the table, take a ho rizontal course. The stick should not, therefore, be finally fastened until the operator is sure of a perfect equilibrium. In firing, it is placed on a table, with the two lower holes downwards; and, after spinning for a second, it ascends with a rapid and noisy circular mo tion. The books of pyrotechny order various orna ments to be appended to it, such as stars and crackers ; but this is impossible, as its nature is so delicate that it will not bear the slightest weight or incumbrance of any kind ; besides which, its duration is far too short for such contrivances, as it does not burn above four or five seconds. We may add, that there is no better test of an artist's accuracy than a good tourbillon, and that a slovenly one need not attempt it.
Of Table Rockets.
This kind of firework is not applicable to public exhibitions, because it is not sufficiently visible at a distance. But it is well adapted for a small number of spectators, and is very ornamental, while it is at the same time a test of the artist's accuracy of work manship. The case is filled and managed in every re spect like the tourbillon, but the composition may be varied in different parts, so as to'produce some changes of appearance during the burning. But it must be
remembered, that whatever composition is introduced at the first end in driving, the same must be done for the others, that both the ends may regularly corre spond in quality all the way to the centre. Supposing it, therefore, to be eight diameters in length, which is sufficient, the first diameter, or inch, if it is an inch case, may be filled with a strong charcoal fire, for which receipts will be found in the table. This may be followed by the same quantity of brilliant iron fire. Both of these may be strong compositions, that the firework may acquire sufficient velocity at the com mencement. Halt an inch, or less, of a flaming and slow fire, without sparks, may then be introduced, which is again to be succeeded by a strong one, to give it an opportunity of recovering its velocity, and that it may be strong when it arrives at the middle, so as to end in a lively manner. The remainder of the case is then to be filled with the same compositions, in a reverse order. The case being then completely closed at both ends, as for the tourbillon, two holes must be bored into it at the extremities, and on oppo site sides, that it may revolve like a wheel. To in sure a sufficient velocity, if that is desired, each of these bores may also be prolonged according to the axis of the case, as in a rocket, but they need not extend inwards above a diameter. It must be primed by a single leader, that both ends may fire at once.
To arrange it for firing, it must be fixed on a short obtuse cone a, (Fig. 9.) like a child's top, taking care that it be perfectly balanced, and that it may be spun like a top on the table where it is to be fired. When that is done, it acts the part of a wheel on the table, with some little variety, arising from its lateral change of place. From the proximity of the spectates to such e firework as this, it will not well of terminat ing with a bounce. But it may enu with a discharge of serpents. This is done by slung a short thick case in the centre, with the opening directed upwards, which may be filled with serpents, and the requisite priming communicating by means of a hole with the middle of the firework. This case must not exceed an inch in length, lest it should overbalance the wheel ; and the serpents, that must not be more than an inch and a half long, may be attached and retained in their place by a bit of thin paper pasted round them and the case together. These fireworks may also be made double, by fixing two rockets transversely on the cone, all the vents being directed the same way ; and in this manner they succeed better, as the balance is more per fectly preserved.