E3 / 9 = drams.
Where E denotes the size of the rocket, in eighths of an inch.
Required the quantity of fuse for a 6⁄8 rocket.
(6 × 6 × 6) / 9 = 4 × 6 = 24 drams = 11⁄2 ounce.
For a 3⁄8 rocket?
(3 × 3 × 3)/9 = 3 drams.
The same weights denote the quantity of stars which the rocket will safely carry: thus an ounce and a half of stars may be put into the head of a 6⁄8 rocket; and 3 drams into a 3⁄8 rocket. Along with the stars is to be put in 1⁄9 the weight of the stars of bursting powder; this may be pure meal powder; or a mixture of 8 meal powder, 1 fine charcoal, well sifted together: or half meal, half grain; thus the quantity of bursting powder for a 6⁄8 rocket will be 24⁄9 = 22⁄3 drams; and for a 3⁄8 rocket, 3⁄9 = 1⁄3 of a dram. It is advisable to keep nearly to these directions, for the weight of the stars; but it is not necessary to be minutely exact; but to ascertain whether the head is too heavy or not, take a table knife, or any thing an inch broad, and lay the rocket flat and horizontally upon it in such a manner that the commencement of the head lies close to the back edge of the knife, the cutting edge lying towards the choke: if the head pitches over, it is too heavy, and some of the stars must be taken out; if it balances, it is correct.
Rockets should be fired from two staples, or two screweyes, fixed in a post, one near the top, the other half a yard below, as in fig. 70. They should never be propped against a wall, a chairback, a gate, or railing, as they might fall, especially on a windy night, after the touch-paper was lit, and before the fuse had caught. Every possible care should be taken in guiding them, as it is too late to think about any mischief they may cause, after they have once started.
In making rockets, it is essential, above all things, to have good nitre and charcoal. The best way, with fresh materials, is to weigh out as much nitre, charcoal, and sulphur, as will make one small rocket. Have the nitre as fine as possible, and dry it over the fire in a 6-inch frying pan, which should be kept for the purpose. If the rocket ascends well you will know that the articles are pure, and you can proceed to use them; but if the rocket does not rise, you may conclude the articles are adulterated, the nitre with salt, or that the charcoal is perhaps merely deal sawdust, burnt in a retort. In this case you must buy the nitre in crystals, and the charcoal in sticks. To powder the nitre, put it into a pipkin, pour on it a little water, set it on the fire, make the water boil, and keep stirring the nitre with a piece of wood, until it is dry and of a fine powder. Charcoal is best ground up in a coffee-mill. It must first be broken into small pieces, about the size of coffee-beans. After being ground it should be sifted through the sieve, the interstices of which are about 1⁄40 of an inch square. To get a correct idea of this size, lay down a line on paper, an inch long: mark it off into eight equal parts by taking the divisions from a foot-rule; then carefully divide one of the eighths into five equal parts. But the best way is to make a square deal sieve, about 8 inches square, and 3 inches deep; then nail on the bottom a piece of perforated zinc, with quarter-inch flemish tacks. The perforations are circular, and should be 20 to the inch, measured diagonally; that is, a diamond-shaped inch, or inch rhombus, consisting of 20 oblique rows, each row containing 20 holes; 20 times 20 = 400. These holes will be about the same size as the ones mentioned in the sieve, because the solid parts between the perforations are also about 1⁄40 of an inch. A strip of deal, 1⁄4 of an inch thick, should be nailed round the bottom, to keep the zinc tight. A square box for a receiver, and another for a lid, should also be constructed, otherwise you will be smothered in sifting charcoal. Making use of the perforated zinc sieve just described, all the charcoal that goes through, fine and coarse, should be used for rockets. It need not be shaken much, but brushed through with a sash-tool. Construct a second sieve with 15 perforations to the lineal inch. Iron borings for gerbes should be sifted through this; use all that goes through. Construct, also, a third sieve with 9 perforations to the lineal inch. Nothing can equal iron and steel for making sparks; but neither will keep long from rusting, after coming in contact with nitre. Coke grains, about the size of pinheads, are a fair substitute. Beat coke into lumps about as large as peas, then grind it in a coffee-mill, and brush it through the 9-perforation sieve; sift the fine dust from what passes through: throw the dust away, and keep the grains. Porcelain may be powdered in an iron mortar, for gerbes; brush through the 9-perforation sieve; sift out the dust with a fine sieve; throw it away and keep the grains. The intense heat of the focus of the choke renders them incandescent; and, from their weight, they are projected to a considerable distance. They are inferior, however, to coke grains, as the latter are to steel filings. We may say, alliteratively; positive, porcelain; comparative, coke; superlative, steel.
Zinc may be obtained with half-inch perforations; a size useful for garden sieves, bottoms of soap boxes, &c.