HEADQUARTERS ARMY OF NORTHERN VIRGINIA, September 20, 1862.

SIR: Since my last letter to you of the 18th, finding the enemy indisposed to make an attack on that day, and our position being a bad one to hold with the river in rear, I determined to cross the army to the Virginia side. This was done at night successfully, nothing being left wagons, and the morning of the 19th found us satisfactorily over on the south bank of the Potomac, near Shepherdstown, when the army was immediately put in motion toward Williamsport. Before crossing the river, in order to threaten the enemy on his right and rear and make him apprehensive for his communications, I sent the cavalry forward to williamsport, which they successfully occupied. At night the infantry sharpshooters, left, in conjunction with General Pendleton's artillery, to hold the ford below Shepherdstown, gave back, and the enemy's cavalry took possession of that town, and, from General Pendleton's report after midnight, I fear much of his reserve artillery has been capture. I am now obliged to return to Shepherdstown, with the intention of driving the enemy back if not in position with his whole army; but, if in full force, I think an attack would be inadvisable, and I shall make other dispositions.

I am, with high respect, your obedient servant,
R. E. LEE, General.

His Excellency JEFFERSON DAVIS, Richmond, Va.

HEADQUARTERS ARMY OF NORTHERN VIRGINIA,
Camp on the Opequon, near Smoketown, September 21, 1862.

Mr. PRESIDENT: As stated to you yesterday, the march of the army toward Williamsport was arrested. General Jackson's corps was turned back toward Shepherdstown, to rectify occurrences in that quarter. Only one or two brigades of the enemy's infantry with cavalry had crossed the river, none of whom had entered Shepherdstown. They displayed a large force of artillery on the opposite bank. General A. P. Hill's division pushed forward, and soon drove them across the river, when this army resumed its march. Only four pieces of artillery fell into the hands of the enemy, which they had carried across the river before they were attacked by A. P. Hill.

I regret also to report that on the night of the 14th instant, when I determined to withdraw from the gap in front of Boonsborough to Sharpsburg, a portion Of General Longstreet's wagon-train was lost. When his division was ordered back from Hagerstown to the support of D. H. Hill, his train was directed to proceed toward Williamsport, with a view to its safety, and, if necessary, to its crossing the river. Unfortunately, that night the enemy's cavalry at Harper's Ferry evaded our forces, crossed the Potomac into Maryland, passed up through Sharpsburg, where they encountered our pickets, and intercepted on their line of retreat to Pennsylvania General Longstreet's train on the Hagerstown road. The guard was in the extreme rear of the train, that being the only direction from which an attack was apprehended. The enemy captured and destroyed 45 wagons, loaded chiefly with ammunition and subsistence.