bridge can be best placed about 100 yards above the remains of the railroad bridge, to enter the arsenal yard through an opening in the arsenal wall..

The bridge at this place will be about 800 feet long, requiring between twenty-five and thirty boats, depending upon the length of the timbers used. The Baltimore and Ohio Railroad Company have in their service 60 canal-boats, and will have upon short notice the number required for a bridge at and near Sandy Hook..

The lift-lock of the canal at Sandy Hook is in good order. The president of the railroad informed me that a sufficient number of ships' anchors to anchor the bridge can be procured in Baltimore. Such bridge can be constructed in a short time, be made very stable and serviceable for all purposes..

A good ferry, supported upon flat-boats (a great many flat-boats can be found on the canal) across the Shenandoah, will be sufficient, as but a small number of men need be placed upon Loudoun Heights..

I visited Williamsport also. There is now a ferry there capable of carrying the four-horse country wagons heavily loaded. A piece of light artillery or 125 men can be taken at once, and in about three minutes. It is held by a wire cable six-eights or seven-eighths of an inch in size. An extensive ferry might be quietly established there be using flat-boats. Plank and light lumber can be found in sufficient quantity at Williamsport. This ferry can be worked by a cable or with poles. The river is now fordable at this place..

I was also requested to give my opinion of the number of men sufficient to occupy Martinsburg. If Loudoun Heights are occupied by 400 or 500 infantry, Keys' Ferry (across the Shenandoah), with a like number of men and one or two pieces of artillery, I think a division sufficient to occupy Martinsburg, or if overwhelmed to make a safe retreat to Harper's Ferry. I would suggest that a strong detachment should be left at Charlestown to secure the rear. While such move is being made Winchester could be threatened by forces from Romney, and when Martinsburg is occupied a junction can be made with the troops at Hancock via Springfield. Harper's Ferry need by occupied by a garrison only. As it would be necessary to leave the artillery in position on the Maryland Heights, I think General Banks should be re-enforced by at least one or two batteries before making such move..

I have the honor, general, to be your obedient servant,.

O. E. BABCOCK,.

Corps of Engineers..

FREDERICK, December 27, 1861-7.30 p. m..

Brigadier General S. WILLIAMS,.

Assistant Adjutant-General, Washington, D. C.:

All is quiet. A scout from Virginia states that the enemy have retired to Winchester; 400 infantry at Martinsburg; 500 cavalry scouting the river; seven guns (34-pounders) in position at Winchester; one 54-pounder..

General Jackson has about 7,000 men-4,000 volunteers, rest militia; twelve light guns, one rifled. Railroad iron of Baltimore and Ohio Railroad piled up at Charlestown and Halltown..

Respectfully submitted..

R. MORRIS COPELAND,.

Assistant Adjutant-General..