Reversed It is easier to manage these horizontally than verti cally, but they may be made in either way. If the ho rizontal construction is to be adopted, a double or tri ple wheel, as formerly described, may be taken, either with two rows of oblique cases, or with these two and an intermediate. horizontal one. But to take the sim plest case, and suppose any two of whatever nature. The last case of the wheel must be made so slow, that its motion may nearly cease before it is burnt out. To distract the attention of the spectators, then, from the wheel and its motions, a gerbe, or some other brilliant case, should be lighted in the centre ; and a little while before that is expended, a leader should be conducted from its case to the second wheel, which is to be so con structed as to fire in the reverse order.
In the vertical wheel the same effect maybe produced in the same manner, two sets of cases being applied to the same felly, Plate CeCCLXX1I. Fig. 25. At the period intended for reversing the motion, a set of lights may be fired upon the spokes. These will continue to revolve in circles for a time, gradually lessening in ve locity as the wheel conies to a state of rest. From one of these a leader is then conducted to the commence ment of the second wheel, the mouths of which have been placed in an order the reverse of the first, so that the wheel will commence to move back again as it were in a contrary order, when the spectators imagine it ex pended. This, and all other similar expedients to ex cite surprise, have a very good effect in this art ; and a thorough-bred pyrotechnist will not despise any kind of quackery that may answer his purpose.
To reverse the motions of wheels, however, suddenly-, it is necessary to have two on an axis, as will be de scribed hereafter.
Of conical horizontal Wheels.
These are described by pyrotechnists, and we must therefore notice them, although their effects are not sufficiently different from others, to render it worth while to adopt them, unless much variety is wanted, as may happen in the case of very extensive exhibi tions. They may be varied in many ways as- to the form of their fires, but a brief description of the gene ral principle N% i I be enough for the artist in fireworks. In these, two, three, or four wheels are fixed upon one vertical axis, but differing in their diameters. Thus there may be a of three or four, forming a cone, or else the smallest may be in the middle, and the largest above and below, or the reverse. They may also be provided with oblique cases tending different ways, and with horizontal ones, or with both ; and it is easy to see how the effects may be varied. The different wheels
may also be fired together, or in different orders of suc cession, on which we need not dwell ; and it is easy to see that their chief differences from the compound wheels formerly mentioned, will arise from the different sizes of the circles of fire.
The Extending and Diminishing Mad.
The effect of this is very good, from the variation in the size of the circle of fire, and it has the advantage that it can be made extremely durable. An ordinary framed wheel being formed in the usual manner, (Fig. 26.) a strong wire or a hoop must be conducted in a spiral di rection from the centre to the circumference ; and it is necessary that a wheel of this nature should he of con siderable dimensions. Short cases are required for it, particularly for the inner turns of the spiral, but the cen tre ones may be of the usual length. The cases are then all attached in regular succession upon the spiral, and connected in the usual manner. When this is fired, it is evident that the circle of lire will gradually increase or diminish as the cases burn in succession from or towards the centre. But as the force of the fire near the axis is not sufficient to put a large wheel into motion, it is bet ter that they should be fired from the outer end, in which case the circle will diminish and still preserve a suffi cient velocity.
This wheel may be varied to advantage by provid ing a circle independent of the spiral. Thus, the large circle will burn, while the spiral line, being fired from the centre at the same time, is gradually extending its circle till they meet. This operation may farther be reversed in this manner. Let two circles of cases be provided on the outer margin of the wheel, and two spirals within; care being taken that there is the same length of case in the spiral as there is in the circle, When the first circle of fire is lighted, the outer end of the spiral may also be fired by the communication of their mouths, or by a joint leader. When that circle is expended, the second will be lighted, appearing to the spectators as a continuation of the same, when the corner of the second spiral will then be lighted from the extremity of the first. In this way, during one half of the wheel, there will appear to be an inner ell-. cle of fire, gradually diminishing ; while, during the remainder, the same inner circle will appear as gra. dually to increase.