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 he 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 electfician, was the first to suggest the use of electricity fur 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 Konstantinoff, of the Russian Artillery, constructed the first clectro-ballistic machine after 'Wheatstone's suggestion, in 1844. But the first really reliable and convenient `chronoscope' was made about 1850, by Captain 'Nay& of the Belgian Artillery.
All chronographs, chronoscopes, etc., for bal listic experiments arc 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 (s). The velocity of the projectile ( r) at the middle point between the screens, is evidently equal to the distance divided by the time The elec tro-ballistic machines most in use at present. are the Le Bouleng,e Chronograph, the Bashfo•th 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 which 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 he 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 ORDNANCE.
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 transforma tion of the potential energy of the explosive, into the kinetic energy of the projectile, is accom panied 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 pro jectile 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 den sity. form of grain, the laws of its explosion (ignition, inflammation, and combustion)—de ducing therefrom algebraic formulas for the burning in air of grains of different shape— size and density of grain, the density of loading. the amount of heat given off, the pressure of the gases. the 'force' of the powder. and many other cireumstances too numerous and technical for this article. Assuming the laws of perfect gases • Mariotte's and Gay-Lussac's laws). he deduces nom a study of all of the above circumstances formula for the motion of the projectile, both inside and outside of the bore, and for the pres sure inside the bore at different points. This different method: but the problem has never been accurately solved.
When firing for ballistic purposes, one or more Vol)lc pressure-gaupc.: are placed in the bore be hind the cartridge. This instrument consists of a piston. moving in a cylindrical channel, and a copper cylinder to be compressed. which is in contact with the piston: the cylinder is central and kept in the axis of the housing by a spring. The change in length of this copper cylinder registers the maximum pressure of the powder. which is read off directly by an instrument specially designed for this purpose. called a calipher.