The filling of such a case requires some precautions ; and, on account of the great force required to drive it firm, a mould is necessary. The length may vary from four diameters to ten or more, according to the views of the artist respecting its duration. When placed in the mould for driving, with the mouth downwards, a plug of wood must be put into the vent so as to fill it up. Half a diameter, or a whole one, of a moderate charcoal fire must be first introduced, that the firework may not break out in all its brilliancy at once, after which it may be filled to the end with the iron composition. If a bounce is required it should be in a separate ma roon case, as it is more manageable and louder; and, from economy, as the gerbe case may serve a second time with the addition of a new vent. The driving requires to be performed with as much force as it is possible to give; fully as great, or even greater than for rockets of the same size ; as the strength of com position necessary for burning the iron effectually is so great, that, if not well driven, it wound burst the case. The larger gerbes, indeed, can scarcely be effec tually driven without the pile engine. When finished, it is advisable to fill up the neck with a blue light or other composition of little show, as the surprise will be the greater when the gerbe comes to take fire.
The smaller gerbes are sometimes called fountains and Chinese fires, but they differ in no respect from the former, except in size and in the nature of the composition, as already noticed. We mentioned that a common choke would answer, if on a very small scale. When they amount to six or eight ounces, however, a better way of making the choke is to drive a diame ter of clay first upon it, which may, after the composi tion is driven, be perforated by a giniblet of the proper size.
In the books of pyrotechny, receipts will be found for making what is called the Chinese spur fire, which are purely imaginary. This is said to be a cold fire, (not a very intelligible term,) and to be made with saltpetre, sulphur, and lamb black. One of the re ceipts gives four quarts of lamp black to a pound of saltpetre and half a pound of sulphur ; a composition which may well be called cold, as it would not burn at all. The truth is, that the Chinese spur fire, sometimes also called flower pots, is merely a gerbe ; and the bright sparks are produced in the usual way by iron. But as the Chinese are great economists in their mate rials, these are made on a very small scale. The com position is simply that of gunpowder, or of saltpetre 75 parts, sulphur 10, and fine charcoal or lamp black fifteen, by weight, well mixed and rammed. A very few grains of iron are introduced at different instances into the composition during the driving, so that instead of yielding a torrent of sparks it gives a red strong flame, with an occasional bright spark. No other di rections can be required for making this flower-pot, which is generally made on a very small scale, so as to admit of being fired, like the fire wheels, and all the other Chinese fireworks, in a room.
We shall not add any thing here respecting the va rious additions that may he made to gerbes, as it would be in a great measure to repeat that which has been said on this subject in other places.
Pots de Brins. Pots di aigrettes. Pots de Sartcissons.
There are three kinds of fireworks described in all the books of pyrotechny, but as the general principle is the same in all, and as there is indeed no great dif ference among them but the name, we have here class ed them all altogether.
The pots de brins, as they are called, are intended to throw serpents, stars, and crackers ; the pots d'agrettes throw serpents only, and the last kind is intended to throw up cases which are half serpent and 'half cracker. These admit of no other variety but in their size; and the management is the same for all.
To make a single case of this kind, it must be of sufficient strength to resist the explosion of the powder, and that is all which is requisite, as no choke is want ed, and as they arc closed at the bottom, and require no driving. They must be considered, in fact, merely as mortars or ieces of ordnance ; and, if of large size, and likely to be wanted again, it is better that they should be made of metal or wood. They need not be much longer than the cases which they are.to contain, if they are intended to fire serpents; hut, if to throw stars, the length will depend on the views of the operator.
The smallest sizes are used by the Chinese to throw serpents, and these serpents do not exceed the sixth of an inch in the interior diameter. A case of three inches'in diameter, and two inches in length is suffi cient for these. But as serpents even of four ounce dimensions may be thrown in the same way, the sizes must be increased accordingly. In loading them, a small quantity of mcaled powder, not above a quarter of an inch in thickness, must first be laid on the bottom of the case, and rammed down tight, to prevent it from being shaken in the carriage. Upon this the serpents are to be placed with their mouths downwards, and so that they may not be too tight in the case, which would prevent them from flying easily' out. As these have sufficient force of recoil themselves, they do not require a large charge in the case, unless it is intended to throw them very high into the air. Should this be desired, the case must have a load of powder in grains first, on which is to be fitted a loose cover of paste board to serve as a wadding. Above this must then be placed as much mcaled powder as will be sufficient to inflame the serpents; and thus the desired effect will be obtained. The same rule may • be attended to, if sancissons, or crackers, are to he used instead of simple serpents. But we must here caution the artist against making these too long, as, if they are, they will over set and fall down. Five diameters are quite sufficient, and even less may answer.