We suppose that all profound and sincere students of military operations are agreed in accepting bridge-trains and skilled pontoniers as among the necessities of grand armies. In proportion as the campaigns which an army is to make are to be conducted on theatres intersected by rivers will be the importance of its bridge-service. Our own country, abounding in rivers of the grandest proportions, will need to be always ready for applying the highest skill and the best bridge-equipage in facilitating such movements as may prove necessary. We accept this as an indispensable part of our organized system of war-_materiel_. Were other evidences lacking, the experiences of the Chickahominy, Rappahannock, Potomac, and Tennessee will perpetually enforce the argument. The generation which has fought the Battle of Fredericksburg, and which has witnessed Lee's narrow escape near Williamsport, is sufficiently instructed not to question the saving virtues and mobilizing influences of bridge-trains. The chief essentials in a military bridge-system are lightness, facility of transportation, ease of manoeuvre in bridge-formation, stability, security, and economy. It necessarily makes heavy demands for transportation; and on this account bridge-trains have frequently been left behind, when their retention would have proved of the utmost importance. Their true use is to facilitate campaign-movements; and while they should be taken only when there is a reasonable prospect of their being real facilites, they should not be left behind when any such prospect exists. It was in response to the demand for easy transportation that the system for India-rubber pontoons was elaborated. Single supporting cylinders of rubber-coated canvas were first experimentally used in 1836 by Captain John F. Lane, United States army, on the Tallapoosa and Chattahoochee Rivers in Alabama. The service-pontoon, as arranged by General Cullum, is composed of three connected cylinders of rubber-coated canvas, each having three compartments. On these pontoons, when inflated, the bridge-table is built, lashed, and anchored. This bridge has remarkable portability, but it has also serious defects. The oxidation of the sulphur in vulcanized rubber produces sulphuric acid in sufficient amount to impair the strength of the canvas-fibres, thus causing eventual decay, rendering it prudent to renew the pontoons after a year's campaigning. The pontoons are required to be air-tight, and are temporarily made partially useless by punctures, bullet-holes, rents and chafings, although they are easily repaired. Hence this bridge, despite its portability, is hardly equal to all the requirements of service, though it was the main dependence in Banks's operations in Louisiana, and was successfully used in Grant's Mississippi campaign.