Loudoun County, Virginia, represent that 9,000 or 10,000 men have been drawn south from Manassas..

The canal is now open to navigation. We were told that boats with coal were near Harper's Ferry yesterday on the way to Georgetown, and that 1,000 tons daily would soon be sent to the town for public and private use..

The opening of the railway is a feasible project, but it will require the whole force of our division to protect the work..

I am, sir, with great respect, your obedient servant,.

N. P. BANKS,.

Major-General, Commanding Division..

WASHINGTON, D. C., December 7, 1861.

Brigadier-General MARCY:.

SIR: I have the honor to submit the following report concerning the facilities for passing troops across the Potomac at Harper's Ferry, Williamsport, &c. At Harper's Ferry the river is about 600 feet wide. The Maryland shore is of difficult access, owing to the canal bank, being nearly 20 feet high and supported by masonry. The Virginia shore is in like manner difficult, owing to the embankments of the arsenal yard, which re supported by masonry..

There is a lock leading from the canal into the Potomac a short distance below the remains of the railroad bridge; at low water usually from 4 to 6 feet, but rises very rapidly on sudden rains to a height of 26 feet. The difficulties of a bridge at this point are the approaches. Passing up the river about 1 mile to the Government dam the river widens a little, but Virginia shore is of a much easier access, and the country road leading from Harper's Ferry to Charlestown can be reached by passing one-half mile over a country road. The Maryland shore at this point must be reached by passing a short distance upon the tow-path of the canal, which is sufficiently wide for one track but difficult for two. There is also a lock leading into the Potomac at this place. .

As directed, I consulted Captain Duane (Colonel Alexander being ill), and learn that the pontoon bridge at the Eastern Branch is serviceable, and can be sent to any point you may direct. The bridge, being of India rubber, is not very stable and is easily damaged..

I would respectfully suggest a bridge supported by canal-boats instead. There is a sufficient number of boats in the canal, and lumber of suitable size can be procured at Baltimore or Cumberland on very short notice. The bridge-builders of the Baltimore and Ohio Railroad can without doubt be procured to do the work. The boats can be locked into the river and anchored in their places immediately..

At Williamsport the river is of nearly the same width as at Harper's Ferry; current not rapid; approaches on either shore of easy access. A small ferry-boat of a few tons burden plies by means of a wire cable from shore to shore. A bridge supported by two flat-boats, such as are found in nearly every level of the canal, would enable from 300 to 500 men to cross at once. There being no lock between Dams Nos. 4 and 5, the boats must be moved from the canal be means of a derrick or like machinery. Plank can be procured at different points on the canal-at.