The proper line for this purpose is that which is call ed jack-line, and it must be stretched as tight as possi ble, lest it should come too near the ground in the middle. Where simple, or alternating plain rockets are fired, the line should cross the direction of the spec tator's sight. If the rocket is to carry a wheel, as i hereafter to be described, the effect is better if it lies in the direction of the line of vision.
To carry the rocket on the line, there must be a perforated piece of wood, or a paper tube of the same length, adapted to it, through which the line is to pass freely, and it. will also insure the facility of flight if the line is soaped or greased. The priming and firing of such a rocket is so simple an affair, as to require no particular directions ; and it is easy to understand what its effect must be. But as a single line rocket is soon expended, and as it is very easy to compound them, since there is no difficulty in procuring sufficient force, it is better that they should be constructed in this man ner. Two rockets may be applied to the same tube, but in reverse order, so that the mouth of the one and the termination of the other are approximated A leader is then conducted from the end of the first to the be ginning of the second, so that when the rocket has ar rived at the end its flight it returns again in a simi lar manner. Pursuing the same principle, four may be used instead of two, or even more, so as to keep up a succession of alternations ; nor is there any other limit to this but the strength and horizontality of the line, because the friction is so small on the horizontal plane, if the arrangements are well made, that a good rocket will carry forwards a great many others.
But the variety of effects which line rockets are able to produce, are not limited to mere alternations. It is easy, for example, to attach to it a bright light of any kind, which may burn as it goes on, and which may be made sufficiently durable to last out three or four alternations. Such a light should be attached in a parallel manner, that it may not impede the motion, and at a sufficient distance from the line to avoid all hazard of burning it. It is also easy to attach a Roman candle to it, (a firework hereafter described) by which means a very pleasing effect may be produced, as it will continue to discharge stars during its flight. Thus, also, it may be caused to discharge serpents, by attach ing these to the cases in the manner already deserweu under the head of sky-rockets.
It is much easier to attach the transparencies to line rockets, as is directed in the books of pyrotechny, than to produce any useful effects in this way. It is easy, for example, to understand how a line rocket may carry the figure of a flying dragon, or a ship. Such an object may be made of transparent cloth or paper, properly stretched upon a frame, and illuminated within, or pro vided with lights, or fires, or crackers, which may be lighted at any time the artist thinks proper, by regulat ing the disposition of the quick or slow match which is to fire them. But the rapidity of the line rocket gene rally destroys the effect of these contrivances ; nor is it very easy to make them sufficiently slow, without a risk of their stopping altogether should any temporary obstruction occur in the line or in the rubbing parts. A dextrous and attentive workman may, however, sur mount these difficulties ; and in that case he has it in his power to produce many pleasing effects. But it is unnecessary to describe particularly how these _trans parent figures may be made and varied, as that may be equally well clone without directions by any ingenious workman.
A line rocket may be varied in a very agreeable man ner by causing it to carry wheels at the side ; but in this case its motion should be slow. The wheels for this purpose are made of single cases, in the manner here after described, and are attached by a spindle to the wooden tube which carries the rocket. 'I hey are fired at the same time as the rocket ; and as one may be placed on each side of it, an alternation may in this way be produced. In this case the line must cross the direc tion of the spectator's sight.
But there is a far superior way of combining the wheel with the line rocket ; only that it requires much management, as it is attended with considerable diffi culty In this case the line must run in the direction of the line of vision, as the intended effect is only to be attained in this manner. The intention is to cause the wheel to revolve round the line while it moves horizontally, and the effect is extremely brilliant. In this case it is better that there should be four or six alternations ; as the length of the wheel is easily made to coincide with that number.