The self-propelled rockets have not been altered in essential features since the remote centuries when the Chinese first used them in celebrations. A cylindrical shell is mounted on a wooden stick and when the powder in the shell burns the hot gases are ejected so violently downward that the reaction drives the shell upward. At a certain point in the air, various signals burst forth, which vary in character and color. One of the advantages of the rocket is that it contains within itself the force of propulsion; that is, no gun is necessary to project it. The illuminating compounds and various details are similar to those of the illuminating shells described in another chapter. At present the rocket is not scientifically designed to obtain the greatest efficiency of propulsion, but its simplicity in this respect is one of its chief advantages. If the self-propelled rocket becomes the projectile of the future, as some have ventured to predict, much consideration must be given to the design of the orifice through which the gases violently escape in order that the best efficiency of propulsion may be attained. There are other details in which improvements may be made. The combustion products of the black powder which are not gaseous equal about one third the weight of the powder. This represents inefficient propulsion. Furthermore, during recent years much information has been gained pertaining to the air-resistance which can be applied to advantage in designing the form of rockets. Besides the various rockets, signal-lights have been constructed to be fired from guns and pistols. During the recent war the airman in the dark heights used the pistol signal-light effectively for communication. These devices emitted stars either singly or in succession, and the color of these stars as well as their number and sequence gave significance to the signal. Some of these light-signals were provided with parachutes and were long-burning; that is, light was emitted for a minute or two. There are many variations possible and a great many different kinds of light-signals of this character were used. In the front-line trenches and in advances they were used when telephone service was unavailable. The airman directed artillery fire by means of his pistol-light. Rockets brought aid to the foundered ship or to the life-boats. The signal-tube which burned red, green, or white was held in the hand or laid on the ground and it often told its story. For many years such a device dropped from the rear of the railroad train has kept the following train at a safe distance. A device was tried out in the trenches, during the war, which emitted a flame. This could be varied in color to serve as a signal and the apparatus had sufficient capacity for thirty hours' burning. This could also be used as a weapon, or when reduced in intensity it served as a flash-light.