which did not meet the high expectations it at first aroused. The moral effect of this new weapon was very great, however, and in the defense of positions against infantry it was very effective. It was a mistake, of course, to pit them against the German field pieces.

In the contest for military superiority among the great powers of the world, the greatest activity, the heaviest expense, and the largest number of experiments are now in the direction of the development of field artillery. The twenty years succeeding the Franco-German War saw practically no transformations of field-artillery material. At the end of these two decades the usual method of correcting defects by remodeling old types had become impracticable. The years 1890-92 marked the end of the old systems, and the beginning of experiments culminating in a general rearmament, which in 1910 was as shown in the table of field guns included in the introductory part of this book.

The result of this rivalry of nations, then, is the present rapid-fire field gun, a single one of which will deliver more aimed shots in a minute than a whole battery of old guns.

It is not intended to treat of material further, and the foregoing brief outline of its development has been given simply because increased efficiency in material means an advance in the science of ballistics, which subject we are about to investigate.

The subject of ballistics is generally treated under three heads-interior ballistics, exterior ballistics, and ballistics of penetration. The ancients, of course, knew nothing of the first, and little more of the third kind of ballistics, but the general laws of exterior ballistics, which have not been affected by the improved machinery of modern times, were somewhat familiar to them.

CHAPTER II

INTERIOR BALLISTICS

Scope.-Interior ballistics is concerned with the motion of the projectile while in the bore of the gun, and includes a study of the conditions existing in the bore from the moment of ignition of the powder charge to the moment that the projectile leaves the muzzle.

Practical Results.-The circumstances attending the combustion of the powder, the pressures exerted by the gases at different points of the bore, and the velocities impressed upon the projectile, all of which may be demonstrated mathematically, are subjects belonging to the study of Interior Ballistics.

The practical results of the study lie in the application of the deduced formulæ which connect the flight and the course of the projectile with the velocities and pressures. By means of the formulæ we may determine the stresses to which a gun is subjected by the pressure of the powder gases, and the dimensions of the chamber and bore, and the weight of the powder necessary to produce in a projectile a desired velocity. The action of different powders may be compared and the most suitable powder selected for a particular gun. The interior pressures at all points along the bore being made known, the thickness required in the walls of the gun to safely withstand these pressures is determined from the principles of the gun construction. And so we see that the amount of powder employed, the velocity of a projectile, the shape and dimensions of a gun are not merely accidental, but are based upon a series of exact investigations, all of which are within the scope of Interior Ballistics.

Action of Powder in the Gun.-Explosives, as we have seen, are substances which, under the influence of some disturbing agency, enter into a chemical reaction accompanied by the production of gases and the evolution of much heat. The powder used as a charge for the gun possesses a certain potential force or latent power. Upon combustion the potential force of the explosive becomes a kinetic or moving force, which is made use of for the purpose of projecting the missile through the air.

The effects of the explosion of the powder upon the projectile and the gun are dependent on the quantity of gas evolved, on the accompanying heat, and on the rapidity of the reaction. It is readily understood that the greater the volume of gas evolved at the temperature of explosion, the greater the pressure exerted on the bore of the gun. The rate of evolution of the gas of gunpowder is known as the velocity of emission.

The pressure per unit of surface exerted by the gases from unit weight of the explosive, the gases occupying unit volume at the temperature of explosion, is called the force of the explosive. The progressive emission of gas from gunpowder produces a propelling effect by causing a gradual increase of pressure on the base of the projectile, which is made use of instead of the shock resulting from a more sudden conversion or detonation.

Heat and Work. The quantity of heat determines the quantity of work that may be effected by the explosion. The projection of the missile from the gun is the effect produced by the conversion of the heat of the explosion into work. The total work that can be performed by the gas from unit weight of the explosive under indefinite adiabatic expansion measures the potential of the explosive.

Adiabatic Expansion. By adiabatic expansion is meant an expansion of the gas in such a manner that it performs work without giving heat or receiving it. In this case, the

heat in the gas is converted into work, the temperature of the gas diminishing.

In order to understand the great propelling power of an explosive, due to the expansion of the gas, some idea of the working power of heat, which is the cause of the expansion of the gas, must be had.

The working power of heat or its mechanical value is measured in thermal units. A thermal unit is the heat required to raise a pound of water at the freezing-point one degree in temperature. The mechanical equivalent of heat is the work equivalent of a thermal unit; that is, it is the work that can be performed by the amount of heat required to raise a pound of water at the freezing-point one degree. For the Fahrenheit scale the M. E. is 778 foot-pounds; and for the Centigrade scale 1,400.4 foot-pounds. In other words, the heat which will raise the temperature of one pound of water one degree Fahrenheit will move one pound 778 feet or 778 pounds one foot; and the heat which will raise the temperature of one pound of water one degree Centigrade will move one pound 1,400.4 feet or 1,400.4 pounds one foot. This gives us some idea then of the value of gunpowder as the means of producing heat.

That which actually occurs when powder is ignited is as follows: In the first place, the powder is converted into a volume of gas greater than that of the powder; in the second place, the heat generated by the explosion expands the volume of this gas, and increases its pressure. The pressure of the gas is equal in all directions. Unless the gun breaks, the expansion will naturally follow the line of least resistance, which is along the bore. The pressure due to the expansion in this direction expels the projectile.

Density of powder is the ratio of the weight of a given volume of powder to the weight of an equal volume of water. In determining density the volume considered is the volume actually occupied by solid powder.

Gravimetric density of powder is the mean density of the contents of the volume that is exactly filled by the powder charge. The air spaces between the grains are considered as well as the solid powder in the charge. The gravimetric density is obtained by dividing the weight of the charge by the weight of water that will fill the volume occupied by the charge. It is evident that, if a solid block of powder of a given density be broken up into grains, the volume occupied by the powder will increase and will be dependent on the form and size of the grains. While the actual density of the solid powder does not change, the gravimetric density will depend upon the granulation.

The Gun.-The gun serves two purposes; first, as the containing vessel for the explosive, and a means of confining the gases in such a way that the pressure of expansion will be exerted upon the base of the projectile; and, second, to give the projectile the proper direction.

So long as the gas continues to exert a forward pressure upon the base of the projectile, it continues to accelerate the motion of the projectile, the velocity of which increases until it passes out of the muzzle, and the pressure on its base ceases.

Capacity of Gun.-The powder for a gun of any caliber and length has the greatest efficiency when in grains of such shape and dimensions that the charge of least weight produces the desired muzzle velocity within the allowed maximum pressure. The powder that produces these effects may be considered the standard powder for the gun.

The maximum pressure is dependent on the initial surface of the powder charge. A powder with greater initial surface than the standard powder, that is, a powder of smaller granulation, will produce a greater maximum pressure and therefore will be a quick powder for the gun, and a powder of larger granulation will be a slow powder.

We would get the greatest possible effect out of a charge