me in a dispatch of 1 p. m. on the 11th instant. This dispatch was received here at 3.30 p. m. of the same day, and reads as follows:

GENERAL McCLELLAN'S HEADQUARTERS,

October 11, 1862.

General GEORGE STONEMAN:

A force of rebel cavalry, supposed to be about 2,000 strong, crossed into Maryland yesterday, at McCoy's Ferry above Williamsport, staid at Chambersburg last night, and left this morning at 9 o'clock, in the direction of Gettysburg. It is possible that they may attempt to recross the river opposite Leesburg. It is possible that they may attempt to recross the river opposite Leesburg. The commanding general directs that you keep your cavalry well out on the approaches in the direction of Frederick, so as to give you time to mass your troops at any point where they attempt to cross. General Pleasonton, with a large force of cavalry, is moving to intercept them, and, if they come in your direction, he will probably be near them. We shall have two brigades of infantry at Frederick to-night. Communicate any movements of the enemy these headquarters from time to time.

R. B. MARCY,

Chief of Staff.

Upon the receipt of the above telegram, I immediately sent out cavalry on all the roads, and stationed, the troops as follows: The Third and Fourth Maine, 600 strong (total), were placed at the mouth of the Monocacy river to guard the Potomac in the direction of Point of Rocks; the Ninety-ninth Pennsylvania and Fortieth New York, 700 strong (total), at White's Ford, and to protect the culvert recently destroyed by the rebels and just completed; the Thirty-ninth Massachusetts at Edwards Ferry, to guard that part of the river and the pontoon-bridge train; the Tenth Vermont, with a section of artillery, to guard the depot of supplies at the mouth of Seneca Creek; the reserve, consisting of the Thirty-eighth and One hundred and first New York and Fifty-seventh Pennsylvania, 700 strong (total), with a section of artillery, and the cavalry not on picket and patrol duties, 250 strong, were stationed at Poolesville, with a vies of moving to any point on the circumference of which Poolesville is the center. No mention was made, in any dispatch received, about the enemy having artillery, and I directed the forces stationed at the various points to attack them when ever seen, as they probably had but little, if any, artillery with them.

The line my small force was guarding is about 30 miles along a river that can be forded by all arms at almost any point, with an enemy in front, against which we guarding, and 2,000 cavalry in rear, liable to cross at one point as well as another, and whom I was officially informed would probably cross in front of Leesburg, if at all.

About 9.30 p. m. I received a telegram from Colonel Ruggles stating that Stuart had said to a paroled prisoner that he intended to recross the river at the mouth of the Monocacy. I did not think this information sufficient authority to warrant me in exposing the rest of the line and concentrating my whole force at the mount of the Monocacy.

Nothing more was heard from the rebels by me until 9 a. m. of the 12th, when I received two telegrams from Colonel Ruggles, one dated 2.45 and the other 4.30 a. m. placing it beyond a doubt that the enemy were on their way toward the mouth of the Monocacy. I immediately ordered the regiment at Edwards Ferry up the river, and started the reserve, under Brigadier-General Ward, with instructions to push on toward the firing, which just then begun, in the direction of the Monocacy, and sent back to hurry ;up the brigade of General Robinson, whom I had written to the night before, instructing him to push on his command as rapidly as possible. He was unable to ;arrive until after the enemy had made good his escape. My cavalry pickets were pushed out to Barnesville and Hyattstown, and also in other directions, and gave me information of the movements of the enemy, but not until after the information was received by telegraph. I will add that both Gen