Now, the wheel being placed directly behind the sun, for example, and very near to it, is to be turned regularly round and strongly illuminated by candles placed behind it. The lines that form the spiral will then appear through the spaces cut from the sun, to proceed from its centre to its circumference; and will resemble sparks of fire that incessantly succeed each other. The same effect will be produced by the star, or by any other figure, where the fire is not to appear as proceeding from the circumference to the centre.
These two pieces, as well as those that follow, may be of any size, provided we observe the proportion between the parts of the figure and the spiral, which must be wider in large figures than in small. If the sun, for example, have from six to twelve inches diameter, the width of the strokes that form the spiral need not be more than one-twentieth part of an inch, and the spaces between them that form the transparent parts, about two-tenths of an inch. If the sun be two feet in diameter, the strokes should be one-eighth of an inch, and the space between, one quarter of an inch; and if the figure be six feet in diameter, the strokes should be one-fourth of an inch, and the spaces, five-twelfths of an inch. These pieces have a pleasing effect, when represented of a small size; but the deception is more striking when they are of larger dimensions.
It will be proper to place these pieces, when of a small size, in a box quite close on every side, that none of the light may be diffused in the chamber; for which purpose it will be convenient to have a tin door behind the box, to which the candlesticks may be soldered, and the candles more easily lighted.
The several figures cut out should be placed in frames, that they may be put alternately in a groove in the forepart of the box, or there may be two grooves, that the second piece may be put in before the first is taken out.
The wheel must be carefully concealed from the eye of the spectator. Where there is an opportunity of representing these artificial fires by a hole in the partition, they will doubtless have a much more striking effect, as the spectator cannot then conjecture by what means they are produced.
It is easy to conceive, that, by extending this method, wheels may be constructed with three or four spirals, to which may be given different directions. It is manifest, also, that, on the same principle, a great variety of transparent figures may be contrived, which may be all placed before the same spiral lines.
In representing cascades of fire, it is necessary to observe, that, in cutting out the cascades, care must be taken to preserve a natural inequality in the parts cut out; for if to save time, all the holes are made with the same pointed tool, the uniformity of the parts will produce a disagreeable effect.
To produce the apparent motion of these cascades, instead of drawing a spiral, a slip of strong paper is to be provided, in which there must be made a great number of holes near each other, and made with pointed tools of different dimensions.
At each end of the paper, a part, of the same size with the cascade, must be left uncut; and towards those parts the holes must be made a greater distance from each other.
When the cascade that is cut out is placed before the scroll of paper just mentioned, and it is entirely wound upon the roller, the part of the paper that is then between, being quite opaque, no part of the cascade will be visible; but, as the wheel is turned gently and regularly round, the transparent part of the paper will give to the cascade the appearance of fire that descends in the same direction; and the illusion will be so strong, as to appear as a real cascade of fire.