Batteries should always be well secured on the flanks, and constantly sustained by infantry or cavalry. If attacked by cavalry, the artillery should keep up its fire as long as possible, first with ball, and then with grape when the enemy arrives within a suitable distance. The same rule will apply to attacks of infantry, except that the fire of solid shot at a great distance is much less effective than against mounted troops. The _engineer troops_ are employed on the field of battle principally by detachments, acting as auxiliaries to the other arms. Each regiment of infantry should have a detachment of sappers armed with axes to act as pioneers, for the removal of obstacles that may impede its advance. These sappers are of the utmost importance, for without them an entire column might be checked and thrown into confusion by impediments which a few sappers with their axes would remove in a very short time. Detachments of engineer troops must also act in concert with the cavalry and artillery for the same purpose as above. In establishing the batteries of artillery, in opening roads for their manoeuvres, and in arranging material obstacles for their defence, the axes, picks, and shovels of the sappers are of infinite value. Fieldworks, bridges, and bridge-defences, frequently have a decisive influence upon the result of a battle, but as these are usually arranged previous to the action, they will be discussed in another place. In the attack and defence of these field-works, the engineer troops play a distinguished part. The consideration of this part of the subject, though perhaps properly belonging to the tactics of battles, will also be postponed to another occasion. We will now discuss the employment of the combined arms on the field of battle. Before the French Revolution, all the infantry, formed by regiments and brigades, was united in a single body and drawn up in two lines. The cavalry was placed on the two flanks, and the artillery distributed along the entire line. In moving by wings, they formed four columns, two of cavalry and two of infantry: in moving by a flank, they formed only two very long columns; the cavalry, however, sometimes formed a third and separate column in flank movements, but this disposition was rarely made. The French Revolution introduced the system of grand divisions composed of the four arms combined; each division moved separately and independently of the other. In the wars of the Empire, Napoleon united two or more of these divisions into a _corps d'armee,_ which formed a wing, the centre, or reserve of his grand army. In addition to these divisions and _corps d'armee,_ he had large reserves of cavalry and artillery, which were employed as distinct and separate arms.