Pyrgi. Movable towers, used by the Greeks in scaling the walls of besieged towns. They were driven forward upon wheels, and were divided into different stories, capable of carrying a great number of soldiers and military engines.
Pyroboli. Fireballs, used both by the Greeks and Romans. They seem to have been the very same as the malleoli.
Pyrometer. An instrument for determining the pressure of fired gunpowder by the registered compression of oil,—invented by Dr. W. E. Woodbridge, and used by him and Maj. Mordecai (U. S. Ordnance Department) in experiments at Washington Arsenal, 1854-55. It consists of a small hollow steel cylinder filled with oil and a piston which is pressed inwards upon the oil by the powder gases. The piston has a small stem projecting inwards, which is guided by a tube in the bottom of the cylinder. A steel point presses against the stem and scratches a line upon it, when the piston is moved. The pyrometer is received by a hollow screw-plug placed in the side of the gun at the point where the pressure is to be taken. This instrument is probably the most accurate and delicate one ever invented for the purpose. In the experiments it recorded certain vibrations in the column of gases, which have been generally neglected by theorists on the subject, but which are of great importance to the life of the gun. It seems unfortunate that an instrument which promised so much should have been allowed to fall into disuse, if not almost oblivion.
Pyrotechny. Is the art of preparing ammunition and fireworks for military and ornamental purposes. (See [Ammunition].) Military fireworks comprise preparations for the service of cannon ammunition, and for signal, light, incendiary, and defensive and offensive purposes. The term composition is applied to all mechanical mixtures which, by combustion, produce the effects sought to be attained in pyrotechny. The preparations for the service of ammunition are [slow-match], [quick-match], [port-fires], [friction-tubes], and [fuzes].
[Slow-match] is used to preserve fire. It may be made of hemp or cotton rope; if made of hemp, the rope is saturated with acetate of lead, or the lye of wood-ashes; if made of cotton, it is only necessary that the strands be well twisted. Slow-match burns from 4 to 5 inches in an hour.
[Quick-match] is made of cotton-yarn (candle-wick) saturated with a composition of mealed powder and gummed spirits; after saturation, the yarn is wound on a reel, sprinkled (dredged) with mealed powder and left to dry. It is used to communicate fire, and burns at the rate of one yard in thirteen seconds. The rate of burning may be much increased by inclosing it in a thin paper tube called a leader.
[Port-fire] is a paper case containing a composition, the flame of which is capable of quickly igniting primers, quick-match, etc. A port-fire is about 22 inches long, and burns with an intense flame for ten minutes.
[Friction-tube] is at present the principal preparation for firing cannon; it has the advantage of portability and certainty of fire. It is composed of two brass tubes soldered at right angles. The upper, or short tube contains a charge of friction-powder, and the roughed extremity of a wire loop, the long tube is filled with rifle-powder, and is inserted in the vent of the piece. When the extremity of the loop is violently pulled by means of a lanyard, through its hole in the long tube, sufficient heat is generated to ignite the friction-powder which surrounds it, and this communicates with the grained powder in the long tube. The charge of grained powder has sufficient force to pass through the longest vent, and penetrate several thicknesses of cartridge-cloth.
Fuzes.—See [Fuze].
Fireworks for Signals.—The preparations for signals are [rockets] and [blue-lights].