In order to perform these various duties with the greatest attainable efficiency artillery must possess two essentials. In the first place, it must be able to project the greatest possible weight of shell in a given time, and in the second it must be capable of rapid movement from one point to another so that it may be rapidly brought into use whenever the need for it is greatest. Now, obviously, the heavier the shell to be thrown, the greater must be the energy of the cartridge, and the greater the energy of the cartridge, the greater the strength (and consequently size and weight) of the gun necessary to withstand the pressures produced upon its discharge. On the other hand, if a gun is to be mobile, it must be as light as possible, both so that it can be moved at the required speed, and also that it can be taken over soft or difficult ground. Mobility and shell-power are therefore naturally antagonistic, the two cannot be combined in the same gun. The modern army, therefore, carries a range of guns, wherein maximum mobility controls one end of the scale and maximum shell-power the other. The former is represented by the mountain gun, firing a shell weighing some ten pounds and capable of being moved with great rapidity over practically any ground that a man can traverse laden, the latter by pieces of ordnance throwing a shell whose weight approximates to a ton, capable of very slow movement over good roads and requiring elaborately prepared positions from which to fire. Suppose, however, that we were to take a six-inch gun, that is to say a gun firing a shell six inches in diameter and weighing a hundred pounds, with a range of say twenty thousand yards. This gun will require a cartridge consisting of about twenty pounds of propellant, to withstand the explosion of which the gun must be made of such massiveness that it will weigh some seven tons. Now instead of requiring so great a range, we determine to be satisfied with a range of six or seven thousand yards. We now find that a charge of only some two pounds of propellant will give us this range, and that the gun can now be built very much shorter and less massive, so that its weight is reduced to a ton and a half. We have retained the same weight of shell, but have sacrificed range to increased mobility, and the fruit of our labours is no longer a six-inch gun, but a six-inch howitzer. But in the process of conversion from a gun, the howitzer has acquired a new characteristic. Owing to its heavy charge of propellant, a gun projectile leaves the bore with great velocity, and consequently the gun requires relatively little elevation to hit a target at any given range. A howitzer, owing to its small charge, requires a far greater elevation. Now a projectile reaches its mark travelling at very much the same angle with the horizontal as when it started on its journey. At a range within the capacity of both, therefore, if fired say at a