In preparing cases for gerbes, it is necessary that they should be made strong; as they would be liable to burst, on account of the strength of the composition, which comes out with great velocity. They should be of the same thickness at top and bottom, and the paper well pasted. Their necks should be long, in which case, the iron would have more time to be heated, by meeting with more resistance in its disengagement, than if the neck was shorter; for then it would be burnt too wide before the charge was consumed. Long necks will throw the stars to a greater height, which will not fall before they are spent. They should rise and spread in such a manner as to resemble a wheat-sheaf.
Gerbes are generally made about six diameters long, from the bottom to the top of the neck. Their caliber must be one-fifth narrower at top than at bottom. Their neck is one-sixth diameter, and three-fourths long. There is a wooden foot or stand, on which the gerbe rests. This may be made with a choak or cylinder, four or five inches long, to fit the inside of the case, or with a hole in it to put in the gerbe: both these methods will answer the same end. In the charging of gerbes, there will be no need of a mould, the cases being sufficiently strong to support themselves. Before this operation is commenced, we must be provided with a piece of wood made to fit in the neck. If this precaution is not used, the composition will fall into the neck, and occasion a vacancy in the case, which will inevitably burst it, the moment the fire reaches the air. A weak composition should be put in at first, to the quantity of one or two ladles full. After the case is filled, take out the piece of wood, and fill the neck with slow charge.
Small gerbes, or white fountains, as they are sometimes called, are usually made of four, eight, or sixteen ounce cases, of any length, taking care to paste, and otherwise make them very strong. Before they are filled, however, drive in clay one diameter of their orifice high. When filled, bore a hole through the centre of the clay to the composition. The ordinary proportion will answer for the vent, which must be primed with a slow charge. Large gerbes are made by their diameters, and their cases at bottom one-fourth thick. The interior diameter of a gerbe is found, by supposing the exterior diameter of the case, when made, to be five inches, by taking two-fourths for the sides of the case, and there will remain two and a half inches for the bore.
Gerbes produce a brilliant fire, and appear remarkably beautiful, when a number of them are fixed in front of a building.
The composition of gerbes is similar to that of the Chinese fire. It is to the cast-iron, which enters into it, that its beautiful effects are to be ascribed. In fact, the composition of Chinese fire differs considerably, as we shall notice, when we treat of it, according to the purpose for which it is employed. It is adapted, for instance, in various proportions of its constituent parts, to calibers of different diameters, cascades, representation of palm trees, as well as for large and small gerbes. The old formula for gerbes is the following.
Composition for Gerbes.
| Meal-powder, | 6 | lbs. | ||
| Beat cast-iron, | 2 | lbs. | 1½ | oz. |
The present formula, as we remarked when speaking of compositions for calibers from three-quarters of an inch to an inch, is saltpetre 1 oz, sulphur 1 oz, meal-powder 8 oz, charcoal 1 oz, and pulverized cast-iron 8 oz.
The vivid and rapid combustion which ensues, when this composition is inflamed, cannot be accounted for in any other way, than that the nitre is acted upon by the sulphur, the charcoal, and the iron; that the gunpowder, during its combustion, raises the temperature to the degree necessary for the decomposition of the nitre by the substances mentioned; that sulphurous and probably sulphuric acid, as well as carbonic acid, are generated, by the union of the sulphur and carbon with a part of the oxygen of the nitre; that the iron undergoes a combustion, both in contact with the nitre and with atmospheric air; and, lastly, that the effect, which characterizes this composition, and other similar compositions, into which cast-iron enters, as in the celebrated Chinese fire, is to be attributed to the iron; and the appearance which iron assumes, when in a state of combustion, is owing to no other cause than its rapid combination with oxygen, by which the metal is oxidized. (See [Iron], in Part I.)