(55⁄8 × 4 × 4)/7 = 90⁄7 = 13 drams.
Suppose a fixed case 71⁄4 inches long, and 5 eighths diameter,
(71⁄4 × 5 × 5)/7 = 181⁄7 = 26 drms. = 1 oz. 10 drms.
[GERBES.]
Gerbes, so called from the French word for wheat-sheaf, which they resemble, are fixed choked cases: they do not show well on wheels. As they contain grains of iron, they must not be under 6⁄8; for private exhibitions a good size is 9⁄8, 9⁄8 internal diameter; 12⁄8 external; 3⁄8 the diameter of the choke; 111⁄4 inches, length of case. Charge the case on a nipple, exactly like a wheel case. It is advisable to put in, first, a little clay, to protect the choke, as the fire, being fierce, would, otherwise, enlarge it, and diminish the ascent of the sparks. Upon the clay drive in a scoop of starting fire, and fill up with gerbe composition. This, when containing iron borings, is termed Chinese fire; the pieces, Chinese trees.
The most magnificent of all, however, is the coloured gerbe. For this, some green, blue, and crimson grains, or small stars must be prepared. They may be quarter-inch cubes, cut as directed for the chopped nitre stars. A far better way, however, of preparing them, is to remove the pin a, of fig. 2, up to c, so that the stars, driven in the tube, will be 5⁄8 inch diameter, and 3⁄8 thick. When these are dry, chop them into 4 pieces, by holding a knife, or chisel, across them, and giving it a smart blow with the mallet. They are harder made this way.
To charge the cases: having driven in the clay, and the starting fire, put in 7 or 8 stars, then a scoopful of fuse No. 1 or 2, then 7 or 8 more stars, and another scoop of fuse; mallet the whole 4 layers down firm, with blows not too heavy at a time, but many times repeated. Then put in 7 or 8 more stars, another scoopful, 7 or 8 more stars, and another scoop, and mallet the 4 layers as before; and so repeat. The two layers and the two scoopfuls may fill up, when malleted in, 3⁄4 of an inch, in a 9⁄8 case.