In firing these wheels, it is usual to place them ver tically, as the effect is then best. But they may also be placed in a horizontal position ; a practice which is chilly adopted when they are attached to some other firework to which they are intended to communicate motion. We shall presently show how they may be varied by compounding them.
Of Horizontal Wheels.
This is a wheel, which, although placed in a hori zontal position, produces a very different effect from the preceding, on account of the peculiarity of its construc tion. But in making it, it must be remembered, that as the force of the cases is very much diminished on ac count of the obliquity of their position, all the composi tion must be strong.
The wheel to carry these cases must be circular, and of considerable size, to give a freer motion ; and the cases for it should also not be less than an inch in dia meter. As many may be disposed on it as it will hold, as it is indifferent whether they are an odd or an even number.
To construct this wheel, a case is first to be attached to the wheel by its middle, at an angle of 45°, and to insure regularity, it is better that such a number of grooves as is necessary should be cut in the felly, at the proper angle. This case being thus applied, a second must be fastened on the next groove, with its mouth in the contrary direction, so as to be opposite, and close to the extremity of the first. Thus all the cases are to be applied in succession ; so that when the wheel is com pleted it presents a zig zag line of rockets surrounding the felly ; each rocket being at right angles to the next, and all the mouths and extremities following each other alternately. The joints must then be primed and at tached in the usual manner.
It is evident that the action of this wheel, when burnt, will be to produce alternately an oblique stream of fire ascending at an angle of forty-five degrees, and one descending in the same manner ; while it will, at the same time, revolve round its axis. But on account of the resolution of forces, its circular velocity is only the half of what it is in any wheel of which the cases are parallel to the plane. Hence the effect is rather Of all the fireworks that have been invented, this is the most beautiful in proportion to its expense, while it is scarcely possible that it should fail, if made even with the most common attention. Being on a very
small scale, it may be fired in the hand and in a room. It does not indeed admit of being made large, as, if the stream of fire is too great, it is apt to burn the eases irregularly where the turns come into contact, and thus destroy the effect.
The lengths of the paper for forming the tubes of these wheels should not exceed a foot or fifteen inches, as they become difficult to fire, and it is better, there fore, to join two or more together, when it is desired that the wheel should burn a long time. The paper must be thin, and must not exceed two or three folds, as it is necessary that the case should burn through every now and then to give vent to the fire. These tubes are pasted up round a wire, and, before filling are pinched close at one end. They must be filled by means of a funnel, and loosely, else they will crack in the bending; but the first trial will soon show the operator how much composition they will bear. The composition for them will be found in the table, and it need not be varied, as, owing to the occasional burning of the case, and the more or less difficulty which the fire finds in issuing, they naturally produce sufficient variety. It must be made very dry before using, as it would not otherwise fall freely down the tube.
When the tube is filled and secured by pinching at the end, it is to be passed round a wooden turned button, provided with a groove to receive it, and with a hole in the middle to receive the pin on which it is to revolve. By means of a little glue, the first turn is fastened to the cylinder or button, and the remaining turns may be seem ed to each other with a little paste. A bit of twine must be used, to prevent them unfolding in the making, which can be taken off when they are dry. As one tube is expended, another may be inserted into it, and fastened by a slip of paper and paste ; and thus the spiral may be prolonged to an indefinite length. These wheels may therefore be made very durable, so as to last as long as any particular firework with which they may be used. When the requisite size is obtained, the whole should be pressed flat, and secured by some trans verse slips of paper.