A Battery with the leaders complete is represented in [Fig. 19].
Gerbes.
8. Gerbes.—This is a species of Fire-work which, from a cylindrical case, throws up a luminous and sparkling jet of fire, which From its partial resemblance of a water-spout, the French have given it the appellation of Gerbe.
Gerbes consist of a strong cylindrical case made of thick paper or paste-board, and filled with brilliant composition, and sometimes with stars or balls placed at small distances, so that the composition and balls are introduced alternately; immediately below each ball is placed a little grained powder. This last kind of Gerbes are more properly called Roman Candles, which we shall describe in the next article. Gerbes are sometimes made wholly cylindrical, and sometimes with a long narrow neck; the reasons for making them with a neck are deduced from rather philosophical considerations: when fired they exert great force on all parts of the case, especially at the mouth, from which it proceeds with great velocity; the reasons therefore deduced for making them with a long neck are—first, that the particles of iron, which enter into their composition, will have more time to be heated, by meeting with greater resistance in getting out than with a short neck, which would be burnt too wide before the charge be consumed, and spoil the effect; secondly, that with long necks the stars will be thrown to a greater height, and will not fall before they are spent or spread too much; but when made to perfection, will rise and spread in such a manner as to represent pretty exactly the form of a wheat-sheaf.
The diameter of Gerbes is generally estimated by the weight of a leaden ball, which the case is capable of receiving; thus we say Gerbes of eight ounces, of one pound, &c. Their length from the bottom to the top of the neck should be about six diameters; the neck being about one-sixth diameter, and three-fourths diameter long. They are filled in two ways, according as they have a neck, or are wholly cylindrical; the cases of the latter kind are closed below, and are filled like those of serpents, but the composition must be put in by small quantities, and rammed very hard; cases with necks are filled from the bottom, but you must be careful, before you commence ramming, to plug up the aperture of the neck with a piece of wood fitted to its diameter, for if this is not done, the composition will fall into the neck, and leave a vacancy in the case, which will cause it to burst as soon as the fire arrives at that part of it.
You must observe, too, that the first ramming or two be of some weaker composition than the body of the case. When filled the plug must be removed, and the neck filled with some slow charge, and capped with touch-paper; a foot of wood is afterwards to be fixed to the Gerbe and well secured, either by a cylinder fixed to the outside of the case, or by having in it a hole, into which the case may be inserted; when either of these methods is employed the foot must be firmly attached.
Sometimes sparks (Article 5,) are introduced during the filling of the cases, but in this case special care must be taken that they are not broken by hard ramming; their number should be regulated by the size of the case, and when carefully used, they produce a pleasing effect, but they are most adapted to such Gerbes as are wholly cylindrical.
The following method of finding the interior diameter of Gerbes is generally employed:—supposing the exterior diameter of the case at bottom (which is usually made somewhat larger than the top) to be four inches, then by taking two-fourths for the sides of the case, there will remain two inches for the bore, which will be a tolerable good size, and from the rules given for the height the same will be about twenty-four inches to the top of the neck. [Fig. 20] represents a wooden former; and [fig. 21] a Gerbe with its foot complete. The composition for filling will be found in the Table, [Section 7].
In ramming large Gerbes an external mould will not be requisite, the cases being sufficiently strong to support themselves.