Wheels of this construction may be made of all sizes, from two ounces upwards to a pound or two ; and the cases must be as strong as those intended for tourbil Ions, as they have a considerable force to bear. This wheel indeed bears a resemblance to the tourbillon in every thing but the central holes, and is exactly the scroll formerly described, with the addition of an axis and centre They may, however, be made of double the length of cases for all other purposes, or of any length from eighteen to twenty.four interior diameters. Thus the half inch wheel will be a foot long. If more, they will still act well, provided that both orifices are fired together.
In driving them, a mould must be used ; as the same hardness and regularity of composition are required as in the tourbillon. The compositions may also be varied, as was described just now for hexagonal wheels ; but in doing this if the wheel is a simple case, perforated at both ends, the artist must recollect that whatever com position is placed at one end, the same ought to be placed al the other ; but in a reverse order, so that the circle of fire may be uniform Yet a variety is also ad raissible, by entirely reversing the quality of the com position at opposite ends ; so that while one orifice is producing a sparkling fire, the other may give a blue or a white flame. We need scarcely add, that the com positions may be selected from the table at the artist's pleasure ; but as this wheel is so short and light, that little force is required to put it in motion, the operator need not be anxious about using strong fires.
In driving this case, one end must first be choked quite close; or what is better, the ends of the folds should be turned in with glue, and beat down square and solid so that there may be as little unnecessary length as possible. The centre of the case must then be measured and marked upon the case and rammer both, and half a diameter must then he laid down upon the latter, on each side of this line. Thus, when the artist has arrived at this point, be will have notice, without farther trial, that the first half of the case is filled. Clay is then to be introduced and beat down with the same degree of force. so as to form one diameter. In the larger cases, indeed, less will answer the purpose, as the only object of this is to form the centre through which the axis is to be inserted. This at least is the
common practice ; but we have found that these wheels answer their purpose equally well without any central clay, while, at the same time, they burn a little longer when made in this manner. Whichever plan is adopt ed, the case is to be completed to the upper extremity, when the ends of the folds are to be turned down with some paste or glue, and driven down solid by means of the rammer.
The case is then ready for boring. To do this accu rately four lines must be drawn upon it, parallel to the axis, at equal distances, so as to divide the circle into four quadrants. By means of a bit of string to suspend to it, the centre of equilibrium must then be found, and a mark made for the hole which is to be bored through it for the purpose of carrying the spin dle. This hole need not exceed a quarter of the inte rior diameter ; and being done, the edges of the paper are to be rubbed down smooth with grease, and a hit of tallow introduced into the hole, to remain there till the firework is wanted for use. The spindle mrist be well fitted to this hole, yet nut too tightly, and polished ; and it must also be provided with a nail head so smooth ly turned inside, as not to check the motion of the wheel should it come into contact with it tiering its revolutions. When it is to be put on the spindle, a small hemispherical button of wood should also be forced on afterwards, to retain the wheel in its place, and prevent it front coming in contact with the sup port, of whatever nature, to which the spindle is to be fixed. These precautions are necessary, at least for the larger wheels of this class; although, in the smaller, they may be dispensed with; and, we need scarcely add, that all these rubbing parts should be well greased.
It is next necessary to bore the case for firing. This is done, as in the tourbillon, at the extremities of the case, and on opposite sides, and on those two lines which are at right angles to the two through which the axis passes. Thus, when the fire issues from these, the case revolves like a wheel. These holes must be equal in diameter to half a diameter of the composition ; and the artist ought here to be reminded, that a set of gim blets, accurately measured from the eighth of an inch upwards, and numbered, must form a part of his esta blishment of tools.