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.

For many years experiments have been made upon the use of the invisible rays which accompany visible rays.  The practicability of signaling with invisible rays depends upon producing them efficiently in sufficient quantity and upon separating them from the visible rays which accompany them.  Some successful results were obtained with a 6-volt electric lamp possessing a coiled filament at the focus of a lens three inches in diameter and twelve inches in focal length.  This gave a very narrow beam visible only in the neighborhood of the observation post to which the signals were directed.  The beam was directed by telescopic sights.  During the day a deep red filter was placed over the lamp and the light was invisible to an observer unless he was equipped with a similar red