Of Compound Spiral Wheels.
These are formed on the same principle as the pre ceding, namely, that of oblique forces. The frame work for this wheel consists of a long nave, capable of carrying two sets of spokes at six inches or a loot dis tance, as the artist may desire. Thus there is a double wheel of fire revolving on one axis. The cases are to be fixed on these in the oblique manner already de set ibcd, but this wheel admits of two principal variations. All the cases may be consecutive and parallel on both wheels, with the fire directed upwards or downwards, if the artist prefers it ; or else they may be reversed, so that one set of fires shall act obliquely upwards, and the other obliquely downwards. Care must be taken, however, that all the recoils may have the same ten dency ; and it is plain that the force of such a double wheel will, with an inclination of 45 degrees for the rockets, be equal to that of a single horizontal one in one plane.
Besides this, however, the effect may be varied by causing the fires of the upper circle to tend downwards, and those of the lower one upwards. Thus they may be caused to cross each other, so as to produce a much better effect than in either of the preceding modes. A variation may be produced even upon this plan, by either actually crossing the rockets themselves, so as to separate their streams of fire, or by bringing the two mouths together from the lower and the higher wheel, so that two oblique and diverging streams shall appear to issue from one point. A horizontal wheel may be added to these to increase the revolving force and vary the effect ; and, as in the former kinds, they may carry additional fixed fires upon the spokes.
This spiral or compound wheel may further be doubled ; or the same axis may early two pairs, each of which pairs being approximated in the manner already described, a very complicated and brilliant intersection of streams of fire is the consequence. There is no danger in these contrivances of wanting force, pro vided the machinery be well made, as the friction does not increase in so great a ratio as the powers of re coil.
Of Diverging Vertical Wheels.
The construction and nature of this contrivance may be easily apprehended front the description of the for mer. The wheel requires, however, to be differently made, as far as the felly is concerned. Provision is to be made for attaching the cases to it, not in tilt. outer margin, but in the flat side of the felly, and on both the opposite sides. For this purpose grooves must be cut in it at all the points where the cases are to be at tached, which may be numerous in proportion to the relative diameter of the wheel and the lengths of the cases. These must be made as accurately as possible at angles with the tangent at those points, which may vary Irons 5 to 20 degrees, according to the degrees of divergence which it is desired that the fire should have. But the angles on one side of the fay must he above the tangent, and on the other below it. Two cases are then to he attached together, one on each side of the felly, with their mouths parallel, and as near to each other as possible, and in the same way the wheel is to be compleRd all round. It must then be printed, and regulated by the leaders in such a manner that both rockets, on opposite sides of the wheel, may burn to gether. That no misapprehension about this may arise, it is plain that such a wheel, when completed, will be double, and that, when looked on, each proxi mate pair of cases will cross each other in the middle. Each extremity of a case on one side will also be attach ed to the result of a following one. Thus, on burning, two circles of fire will be formed, one outside of the other ; but owing to the obliquity in the positions of the cases, these, during the revolution of the wheel, will form two sets of spiral curves, producing a very pleasing effect. It is evident that, if the divergence of the cases was as great as 45 degrees, the force of revo lution would be equal to that of a single wheel. But as the whole angle need not exceed 10 or 20, the power will be far greater, and the velocity of course more Considerable than that of any single wheel, whose cases are tangents to the circle.