The frames to carry wheels are conveniently made of ash hoops, connected by spokes inserted into a nave. It is also proper that the margin of the wheel, whether it be a circle or a polygon, which are equally convenient, should be of considerable weight. If well balanced, as it ought to be, and well centred, the motion is almost equally free as in a lighter wheel ; while it has the advantage of acting like a fly, maintaining the impulse which has been communicated to it at first. This is particularly necessary to attend to; because as the cases burn out in succession, the balance becomes injured, and when the weight of these is small in proportion to that of the wheel, their effect in disturbing its velocity is less felt. The centres should be at least two inches deep where they receive the axis, to prevent any irre gular motion, made of polished metal, well greased, and with a true axis. The axis may be either fixed into the supporting post by means of a screw ; or, what is better, may be kept there ready, and the wheel put on it, and screwed by a linch pin or a nut. For the smaller classes of single cased or spiral wheels, a sufficient centre may be obtained through driven clay or wood, pro vided it be well greased, and the pins that are to serve for axes are best made of brass or iron, in the form of a thumb screw, with a flat cutting thread, so that they may be easily fixed in the wood and withdrawn, to be preserved, when no longer of use. These general rules will direct the artist for the management of all the va rieties of wheel movements where their own force is trusted to for the motions.
But where wheels are of very large diameters, or the circle of fire is meant to be wide; or when, from the cases being disposed in an oblique manner, the velocity is so much diminished that the effect arising from their revolution is injured, it becomes necessary to call in the aid of the mechanical powers, and to apply revolv ing forces to them. This also is necessary in the use of illuminated revolving cones or globes, in the coun termovements of suns and other complicated figures, as well as in all those wrwlzs where it is necessary to give motion to transparent paintings. The means of doing all this, which may be varied in an infinitude of ways, both as it relates to the direction and the velo city, cannot be too simple ; and it is also proper that they should be concealed from the spectators, who are apt to undervalue a firework that does not seem en dowed with the property of self-motion. It is, how ever, unnecessary to describe matters so simple as a toothed wheel and pinion or screw, which, disposed in various ways, may be made to produce any effects, whether as to direction or velocity. By using the pinion at 45°, the horizontal and vertical directions may be combined, or the one substituted for the other. If the work is of such a nature, as happens in architec tural fireworks, as to conceal the operator, this ma chinery may be turned by hand ; if otherwise, a weight running over a pulley is easily applied to a wheel or crank for that purpose.
It would be as endless as it is unnecessary to describe the various forms of frames that may be required ; as they must differ with the objects of the fireworks ; and we need not, therefore, attempt to direct what an artist can be at no loss in accomplishing. But wherever
cases are to be fixed, whatever their natures may be, proper sockets of metal should be provided for receiv ing them, or pins and hooks, to which they may be fastened. The artist, must, however, take care that no bounce or cracker be allowed to explode within a me tal socket, on account of the risk of injury to the spec tators. It must also be recollected, that whatever means are used for fixing these, although it should even be the lights for figured illuminations, they must be firm ; as the explosions of the leaders and primings might otherwise displace them, and ruin the effect of the work. Larger cases for gerbes, Roman candles, and other fire works, that exert much strength, are best secured by having a metal ring with two ears, or Haunches, which may be nailed down to the stands in which they are to be fired.
Transparencies form a very essential part of all fire works on the great scale, as they arc very ornamental, cover large spaces, consolidate all into one great mass, and are very economical in saving many burning fire works.
For these the pyrotechnist must depend chiefly on the painter; particularly when emblematical figures are introduced, or where architecture is to be repre sented. In works on this scale, the great mass of the firework consists of transparent painting, and the burn ing fires are only its ornaments, to be lighted and re newed in various parts of the building in succession, so as to keep up the effect for a great length of time. All these paintings are executed on stout white linen, such as that used for window blinds, if on a large scale, and with the same kinds of oil-colour diluted with tur pentine; the whole being rendered transparent, where necessary, by a coating of drying oil or varnish, and darkened, where required, by means of black paint. These paintings must be properly stretched on separate frames, so that they may be kept tight, and easily re placed, and taken away front the greater frame-work, to which they are attached.
In smaller transparencies, particularly where they are not likely to be wanted again, or where only some temporary inscription or emblem is necessary, paper may be used ; but it must be strong enough, and pro perly secured by means of strings stretched tight across it. In certain kinds of wheels, representing suns and stars, transparencies may be turned to great account by using more than one, and communicat ing motion to them by means of the revolving ma chinery. Thus, for example, if one transparency, formed by bright rays, is covered by another behind, so calculated by its colouring as to intercept the light by fits while it is revolving, many splendid effects can be produced. In the same manner, two or more such circular transparencies may be made to revolve in dif ferent directions ; while, being painted with different colours, a great variety of showy appearances may be caused at a very cheap rate. But we have not room to give designs or farther descriptions of these con trivances; while we may refer for them to books in every one's hands, viz. Hooper, or Hutton's Recrea tions.