Page:The New International Encyclopædia 1st ed. v. 02.djvu/497

BALLISTICS. jectile. The gun itself was mounted as a pen- dulum, weighing 10,500 pounds, by suspending it from a horizontal axis by means of rods. The remarkable closeness of the results from these two pendulums furnished a conclusive proof of the theoretical correctness of the method em- ployed.

In using the second method, a great variety of ballistic machines h.ave been invented for measuring the very short interval of time re- quired for the projectile to pass over the short space between two given points of its trajectory. In 1767, an Italian instrument-maker, named Jlattei, invented a very simple machine for this purpose, consisting of a large paper cylinder revolving around a vertical axis. The principle wa.s this: If a musket-ball be fired horizontally through the axis of the cylinder at rest, the two bullet-holes will be 180 degrees apart on the surface of the cylinder; if, however, the cylinder be revolving at a known rate, the two bullet-holes will be more, or less, than 180 de- grees apart, depending on the direction in which the cylinder is revolving, the difference measur- ing the distance a point on the surface of the cylinder has moved, while the projectile was traveling a distance equal to the diameter of the cylinder. A simple arithmetical calcula- tion gives the desired velocity. This machine was also extensively used, in improved forms, by subsequent investigators with good results. With it General d'Antoni determined the initial velocity of musket and rifle balls with consid- erable accuracy; the French Colonel Grobert (1803) constructed a modified form of the ma- chine, which was used by members of the French Institute deputized to experiment with the muzzle velocity of musket-balls ; Dr. Gregory (1818) gave the machine a more rapid and regu- lar motion by kitroducing coiled springs and cog-wheels. Colonel Dobooz, of the French Ar- tillery, invented (1818) a machine for measur- ing velocity depending upon the principle of the law of falling bodies. A movable screen sus- pended by a cord which passed over two pulleys and down immediately in front of the muzzle, was the falling body; near the movable screen was a fixed scrfeen; two marks were made on the screens at the same height from the ground. When the gun was fired and the cord was cut at the muzzle, the movable screen fell, and both screens were pierced by the bullet (almost) simultaneously. The space through which the screen had fallen when the bullet reached it was shown by the two bullet-holes relatively to the two marks on the screens.

With the methods now in use, the error in determining the time which the projectile re- quires to pass over, say 150 feet of its path, must be less than one fifteen-thousandth part of a second if we are to have results correct to within one foot per second, when using a velocity of about 1500 feet per second. It is only by using electricity that such great accuracy _can be obtained, and all the machines now in use are of this character. There are a great many of these machines differing from each other chiefly in the manner of making a record. Wheatstone, the celebrated English electrician, was the first to suggest the use of electricity for this purpose. At about the same time ( 1840) , Prof. Joseph Henry, of the Smithsonian Institu- tion in Washington, proposed a new method for determining the velocities of projectiles, depend- ing upon the breaking of electric currents. Cap- tain Konstantinoft', of the Russian Artillery, constructed the first electro-ballistic machine after Wheatstone's suggestion, in 1844. But the first really reliable and convenient 'chronoscope' was made about 1850, by Captain Nav6z of the Belgian Artillery.

All chronographs, chronoscopes, etc., for bal- listic experiments are constructed on these prin- ciples: Two vertical screens are placed perpen- dicular to the plane of fire; one near the gun, and the other a certain distance (s) farther from the gun; each of these screens is connected elec- trically with the ballistic machine, these connec- tions being severed, in succession by the projec- tile as it passes through them ; the machine records these interruptions, giving the time be- tween them, which is the time ( t ) taken by the projectile to pass over the distance (.5). The velocity of the projectile {v) at the middle point between the screens, is evidently equal to the distance divided by the time {V The electro-ballistic machines most in use at present, are the Le Boulenge Chronograph, the Bashforth Chronograph, and the Schultz Chronoscope.

This brief and incomplete sketch brings us down to the status of the science at the present day. For the proper presentation of the subject, a large volume scarcely suffices. The manu- facturer of guns makes a thorough study of the purposes for ^^'hich his gun is intended, and builds his machine strong enough to perform the work required of it. In general, it may be said that the great desideratum of the artillerist is to give the projectile a low maximum pres- sure long sustained, that is, he wants a slow- burning powder and a very long gun.

For making a popular study of ballistics it is best to take a certain gun, already constructed, and regard it as a thermodynamic machine, in which the potential energy of the explosive is to be converted into the kinetic energy of the projectile. In planning a new gun for any pur- pose, ballistic considerations are of first im- portance, but this belongs more properly to the subject of Ordnance and Gun Construction. See Ordinance.

The two quantities which may be actually measured experimentally, in the study of ballis- tics are the pressure in the bore of the gun due to the combustion of the charge, and the velocity thereby given to the projectile; the former belonging to interior ballistics, and the latter to both interior and exterior. The transformation of the potential energy of the explosive, into the kinetic energy of the projectile, is accompanied by an enormous pressure on the walls of the gun and the base of the projectile, which pressure we require to know, in order to guard against danger to our machine (the gun), as well as to determine the work which our projectile will accomplish. For lack of suitable measuring instruments, pressures were formerly inferred- from the resulting injury to the gun.

The gun being built, the ballistician makes himself thoroughly acquainted with the powder he is going to use, taking into consideration its composition, specific gravity, gravimetric density, form of grain, the laws of its explosion (ignition, inflammation, and combustion) — de-