The name fire jets is given to all rockets loaded solidly, so that there is no air chamber, which operate without leaving the place where they are fastened. Of this sort are the fixed suns, turning suns, and those designed to imitate with fire the play of spouting water. (PL VII, figs. 3 and 4.)
The thickness commonly given to the case for a fire jet is one-fourth of its interior diameter when a brilliant fire effect is intended, but only one-sixth of this diameter for Chinese fire.
The space left for choking should be filled with string, to support the case while it is being loaded.
Each charge when compressed should occupy a height equal to one-half the interior diameter of the case. From 15 to 20 moderate blows of the mallet are struck in the tamping of each charge. The variation is proportioned according to the size of the particular jet. The mallet used should be somewhat smaller than that employed in the loading of rockets having the same diameter.
The case is closed with a plug, unless it is to be used for a turning sun, or a pot i aigrette.
Before priming, the throat is filled with the composition used in loading. The powder is merely pressed into the empty, space with the point of a knife; it is not rammed down. It is necessary thus to fill the space, since otherwise there would be a risk of explosion from dilation of the air inclosed within it.
Since it may happen in connection with large jets that the throat will burn too soon, it is expedient that it be reinforced by ramming into it potter's clay. A jet thus equipped casts its fire much higher.
Such jets may be pierced with two holes near the throat, in order to make them spout fire from three orifices at the same time. The jets produce a beautiful effect when Chinese fire is employed, and are excellent for purposes of decoration.
The composition for sparkling fire is particularly adapted to the representation of playing fountains and of waterfalls, since the fire is thrown high and scintillates.
Jets intended to represent by their fire falling sheets of water should not be choked. For this use the jets are placed horizontally very close one to another.
When the artificer has weighed out the various materials required for the making of Chinese fire, he passes the charcoal and saltpeter three times through the horsehair screen, in order to mix them thoroughly. The iron sand is slightly moistened with brandy, so that the sulphur flour will adhere to it, and the sand and sulphur are then mixed. This mixture is afterward spread out over the combined charcoal and saltpeter, and the whole mass is thoroughly blended. In the making of the composition for jets, when sand above the second order is used, the mixture is moistened with brandy to a point at which it begins to swell. It is then kneaded until the humidity penetrates throughout the whole substance. It should be noted, however, that if the mixture be too moist, it will not give out flowers at the time of its discharge. The object of such moistening is merely to retain the distribution of the sand throughout the composition while it is emptied into the case. This is effected by increasing the tenacity of the parts. Without such precaution, the sand by reason of its weight would tend to mass at the bottom of the case when the composition is poured in, and this tendency would be especially marked for the larger jets.
Such moistening of the composition is not required for rockets, and in fact should be avoided, inasmuch as the humidity would so weaken the driving force as to prevent ascension; while, if they were kept until the composition became thoroughly dry, they would explode on being fired.
It must be observed that the need for moistening the entire composition to be used in jets does not exist when the sand used is of the first three orders.