intention of attempting the passage of the Potomac south of this city, near Aquia Creek, where they are erecting strong batteries, or at some other point. I will recommend to you the utmost vigilance, and that you continually bear in mind the necessity of securing your retreat towards Rockville should you be unable to prevent the passage of the enemy.
General Banks will be instructed to move up to your support in case of necessity, and will also be instructed to effect his retreat in the same direction in conjunction with you should it become necessary. It is still my which that the enemy's passage and subsequent advance should be opposed and retarded to the utmost of your ability, to give me time to make my arrangements and come up to your assistance.
A general order has been issued merging the Departments of Northeastern Virginia, the Shenandoah, and Baltimore into the Department of the Potomac, under my immediate command.
Steps have been taken which will secure us a large re-enforcement during the coming week.
* * * *
Very respectfully, your obedient servant,
GEO. B. MCCLELLAN,
Major-General, U. S. Army, Commanding Department.
HEADQUARTERS CORPS OF OBSERVATION,
Poolesville, August 19, 1861-1 p. m.
Major General GORGE B. MCCLELLAN,
Commanding Division Potomac, Washington, D. C.:
GENERAL: Your letter Numbers 1, of yesterday's date, is just received. I have made and caused to be made the most careful examinations practicable of the opposite side of the Potomac in front of my position, and believe that the only force in the immediate vicinity is a regiment of Mississippi troops at Leesburg and one of Mississippi or South Carolina troops on Goose Creek. The enemy appear to be throwing up additional entrenchments about 3 1/2 miles back from Edwards Ferry, on the Lessburg, but good for nothing for offensive operations. These works were commenced previous to the battle at Bull Run, and are now being extended, according to report. These works might be reached by shot from rifled gun planted on the heights above Edwards Ferry.
The troops of this command are now posted as follows, commencing on the right: Three companies of the Second New York Militia [Eighty-second Volunteers] are stationed at the mouth of the Monocacy, with picket thrown out 2 miles above and the same distance below, connecting above with pickets of the Twenty-eighth Pennsylvania, of General Banks' command. This outpost is supported by the remainder of the Second Regiment, 300 strong only, stationed one-half mile from Poolesville, on the Monocacy road. A picket of cavalry patrols the vicinity of the Monocacy. the Tammany [Forty-second New York] regiment (Cogswell's) is stationed 1 mile from poolesville, on the road to Conrad's Ferry, and has four companies detached to watch that ferry. The strength of this regiment is 531 in camp, besides the four companies on outpost. The outpost at Conrad's throws out pickets to meet those from the Monocacy above and those from Edwards Ferry below. The Minnesota regiment (Gorman's), 788 strong, is stationed 2 1/2 miles from