Still another arm of the service was the pontoniers, whose duty it was to bridge non-fordable rivers. They were armed and drilled as infantry, but only for their own protection. Their specialty was laying and removing pontoon bridges. A pontoon train consisted of forty to fifty wagons, each carrying pontoon boats, with plank and stringers for flooring and oars and anchors for placing. In laying a bridge these boats were anchored side by side across the stream, stringers made fast across them, and plank then placed on the stringers. Every piece was securely keyed into place so that the bridge was wide enough and strong enough for a battery of artillery and a column of infantry to go over at the same time. The rapidity with which they would either lay or take up a bridge was amazing. If undisturbed they would bridge a stream two hundred yards wide in thirty minutes. They bridged the Rappahannock at Fredericksburg under fire on the 12th of December, 1863, in a little over an hour, losing heavily in the act. Having now given some account of the organization of this great human fighting machine, it will be proper to show how it was handled. For this purpose there were four staff departments, namely, the adjutant-general's, the quartermaster-general's, the commissary-general's, and the ordnance departments. The first named was the mouth-piece of the army. All orders were issued by and through that officer. It was the book-keeper of the army. Each subdivision of the army had its adjutant-general down to the office of adjutant in the regiment, who was charged with issuing all orders, and with attending to their execution. He was secretary, so to speak, of the commanding officer, and his chief executive officer as well. Extraordinary executive talent and tireless energy were required in these positions. The adjutant must be able at all times to inform his chief of the condition of every detail of the command whether an army corps or regiment, exactly how many men were fit for duty, how many sick or disabled, and just where they all are. In fact, he must be a walking encyclopaedia of the whole command; added to this he was usually chief of staff, and must be in the saddle superintending every movement of the troops. Always first on duty, his work was never finished.