Memoranda in reference to crossing.

FEBRUARY 8, 1863.

January 15 there were at Baltimore fifty-eight boats, thirty-six feet long, six feet beam, three feet deep, or of two-thirds greater capacity, nearly, than the common pontoon. These boats were intended for a double roadway bridge, the floor timbers six inches by eight inches and thirty-six feet long, the floor plank two inches thick. At that time the bridge was not supplied with anchors, cordage, &c. If to be used, these should be obtained at once. This material would form a heavy, substantial bridge; too heavy to be trown across a steam very rapidly, if the boats were separate. If the bridge were put together at the mouth of the river, it might be towed up by steam-boats, at what rate the officers of the navy should be able to judge. A similar bridge of pontoons, the boats being thirty feet long, would tow somewhat easier. If the navy thought they could not tow the bridge as a whole, the boats all towing sidewise, perhaps they might tow it in parts, the boats endwise. These parts could be put together more rapidly if the common pontoon was used - an hour should be enough for the pontoons. The officers on picket along the river should report any sunken canal-boats or other obstructions from Port Royal up. If the bridge was towed at the rate of four miles an hour, it would take twelve hours between Urbana and Port Royal, three hours more to Skinker's Neck, and two more to Seddon's. If it passed Urbana in the daytime the enemy might, perhaps, get notice of it by way of West Point, or White House and Richmond, in four or five hours. The best time for the bridge to reach its place would be about an hour before daylight. This would require passing the most crooked part of the river in the night, and passing Port Royal about 11 p. m. The alarm might, perhaps, be given at this time by the enemy's pickets at that place. If the cause of the alarm was sent at once to Skinker's Neck, the enemy would have an hour and a half to get guns into the works to prevent the passage of the bridge and steamers. It would then be much better that the navy should take towns so small that they could move them at the rate of six miles an hour; then the bridge might reach any point before the cause of the alarm was known at that point, unless the enemy have a telegraph or very good signals. A part of the pontoons go without beams or floor planks, so as to be used at once for crossing troops to cover putting the bridge together. The division to cross to cross in these boats should be drilled at entering and leaving the boats. If the navy will guarantee to have the boats at the chosen point at a given hour; if this was done so rapidly that the bridge was at its place before the enemy had made any preparation to meet it; if the party thrown across in boats was able at once to cover the construction of the bridge, we should probably get the bridges towed by the steamers across in a time less by two or three hours than that necessary to unload the boats on the bank of the river, put them in the water, and then throw the bridge. On the other hand there would be the chance of failure from the steamers being driven back by the enemy's fire, or from delay by getting aground, giving t get ready at the best points for crossing. It is beheved that a good pilot can take seven feet of water up to Skinker's Neck, and five feet to Seddon's, both at low water; at high water two feet more.

C. B. COMSTOCK,

Lieutenant of Engineers and Chief Engineer Army of the Potomac.

(Handed General Hooker.)

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