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