For the use of an army of a hundred thousand men about to march forward against an enemy, all the parallel roads within a space of at least ten miles are needed, and the more of them there are the better, since the columns can thereby be made shorter, and the trains be sent by the interior roads. Where a sufficient number of parallel roads exist, available for the army, it is usual to put about a division on each--sometimes the whole of a corps--according to the nature of the country and the objects to be attained. We will attempt to illustrate the march of an army by columns in the following diagram. [Illustration] Suppose that E and F are two towns thirty miles apart, and that there are road connections as represented in the diagram. The army represented by the dotted line A B, wishes to move to attack the army C D. Cavalry, followed by infantry columns, would be sent out on the roads E M N and E G I, the cavalry going off toward P and K to protect the flanks, and the infantry taking position at I and O. Meantime another column, behind which are the baggage trains, covered with a rear guard, has moved to L. If the three points I, L, and O are reached simultaneously, the army can safely establish its new line, the baggage trains are entirely protected, and the whole country is occupied as effectually as if every acre were in possession. The formation of a marching column varies according to circumstances, but is usually somewhat as follows, when moving toward an enemy: [Illustration: Skirmishers. Advance guard. Brigade of infantry. Battery of artillery. Main body of infantry. Main body of artillery. Ambulances and wagon trains Rear guard.] The dots representing the ambulances and wagon trains do not show the true proportion of these to the rest of the column, and it cannot be given except at too great a sacrifice of space. They occupy more road than all the other parts of the column combined. With the advance guard go the engineers and pioneers, to repair the roads, make bridges, etc. The difficulties and dangers attending a _flank march_ can be made apparent by a diagram: [Illustration]