Besides their duties with the Field Army, Engineers are as necessary as ever for the conduct of Sieges, and the defence of Fortresses, in which services they have constantly been employed for centuries. 4. INFANTRY Infantry, now the principal Arm, has in modern times recovered the place which it held in the armies of the Ancient World, but lost in the Middle Ages when Horsemen were the Men-at-Arms, or the only fighting men worth considering. Infantry has for three centuries formed the bulk of every army, being the easiest to raise and train, and the cheapest to equip and keep up, as well as the most useful, of all the Arms. On Infantry falls the brunt of the fighting, and the greatest toil in marching, while it endures the hardships of a campaign better than the mounted Arms. It can be used for attack or defence, in close or extended order, on any ground, and in any weather. Infantry can fight with its _fire_, at a distance from the enemy, like Artillery, as well as by _shock_, at close quarters, like Cavalry. But Infantry is slow in movement, and without Cavalry cannot ascertain the operations of the enemy, and will therefore be ill-directed in its own; it is helpless in pursuit, and unable either to complete a victory or cover a retreat. The action, too, of Infantry fire is limited to the range of the rifle and the effect of the bullet, so that it finds in Artillery a useful auxiliary, owing to the greater effect of fire from guns, and the distance at which they can act. Hence Infantry is greatly assisted in its fighting by associating it with Cavalry and Artillery, just as Cavalry is aided by association with Artillery. It is essential, therefore, that not only every Army, but every Body of Troops which may have to fight independently, should have a due proportion of all Arms. This is the reason for organizing Armies in the _higher Formations_, provided with more than one Arm, as contrasted with the Units composed of _one_ Arm only. The latter, however, are the basis of the higher Formations, and their composition and strength must be considered before describing how they are grouped into larger bodies. Therefore the Organization of the Units of each Arm will form the subject of the next chapter. CHAPTER III ORGANIZATION OF THE UNITS OF EACH ARM The formations in which each Arm is independently organized constitute the _tactical units_ of an Army. Their strength and organization are intimately connected with the way in which they are used in fighting, and have varied little since armies first became regularly organized. The general composition of these Units of each Arm in modern armies will now be described, beginning with Infantry, the principal Arm. 1. INFANTRY