The transportation of supplies is limited by the ability of the Government to provide trains, and by the ability of the army to protect them; for large trains create large drafts on the troops for teamsters, pioneers, guards, etc. An army train, upon the most limited allowance compatible with freedom of operations for a few days, away from the depots, is an immense affair. Under the existing allowances in the Army of the Potomac, a corps of thirty thousand infantry has about seven hundred wagons, drawn by four thousand two hundred mules; the horses of officers and of the artillery will bring the number of animals to be provided for up to about seven thousand. On the march it is calculated that each wagon will occupy about eighty feet--in bad roads much more; consequently a train of seven hundred wagons will cover fifty-six thousand feet of road--or over ten miles; the ambulances of a corps will occupy about a mile, and the batteries about three miles; thirty thousand troops need six miles to march in, if they form but one column; the total length of the marching column of a corps is therefore _twenty miles_, even without including the cattle herds and trains of bridge material. Readers who have been accustomed to think that our armies have not exhibited sufficient energy in surmounting the obstacles of bad roads, unbridged streams, etc., will be able to estimate, upon the above statements, the immense difficulty of moving trains and artillery. The trains of an army have been properly denominated its _impedimenta_, and their movement and protection is one of the most difficult incidental operations of warfare--particularly in a country like Virginia, where the art of road making has attained no high degree of perfection, and where the forests swarm with guerillas.