No. 10. Report of Brigadier General John P. Hatch, U. S. Army, commanding First Division, of the battle of South Mountain.

MIDDLETOWN, MD., September 15, 1862.

MAJOR: I have the honor to submit the following report of the operations of my division on the 14th instant, from the opening of the action on that day to the time I was compelled to leave the field by a painful, though not dangerous, wound:

On leaving the turnpike to take up position, Gibbon's brigade, the largest in the division, was detached by order of Major-General Burnside, leaving but three brigades, the total effective force of which could not have exceeded 3,500. The Twenty-first New York, under Colonel Rogers, was first thrown, deployed as skirmishers, with orders to move up a ravine leading to a low place on the crest of the mountain on my right. The Thirty-fifth New York, under Colonel Lord, was also deployed as skirmishers, and directed to move to the crest of the mountain, connecting on the right with the Twenty-first, and covering the whole front of the division. The remainder of Patrick's brigade was moved to the front, as a support for the two regiments deployed as skirmishers. Through some misunderstanding of the order, Colonel Rogers's regiment, instead of going to the point designated, moved up the mountain in front of the division. Of the further movements of this regiment I have no information up to the time of my leaving the field.

The error of Colonel Rogers being discovered, the Second U. S. Sharpshooters, Colonel Post, were detached from the First Brigade, and proceeded up the ravine to the point indicated. I have received no report from this regiment, but have been informed that it came early into action and rendered very important service during the day. The Thirty-fifth New York, supported by two regiments of Patrick's brigade, moved very slowly up the side of the mountain, followed by the First Brigade, commanded by Colonel Phelps, and the Second, General Doubleday, in line of battle of battalions in mass.

The cause of the delay in the first line can best be explained by General Patrick, whose report, I suppose, has been submitted to the present commander of the division.

To ascertain the cause of the delay, I proceeded to the summit of the mountain, where I was only able to find the Thirty-fifth New York. The two supporting regiments were not to be found. The Thirty-fifth New York was then advanced, supported immediately by the First Brigade. The enemy was found posted behind a fence at the edge of a wood, through which our attacking column was advanced, deployed in line of battle. The firing was very heavy, the enemy making a desperate resistance, and our troops advancing with determined courage.

After about fifteen minutes of heavy firing, a charge was made by the First Brigade, which succeeded in gaining and taking possession of the fence held by the enemy; but the resistance of the enemy being so much more determined than had been anticipated, Doubleday was ordered up to support the First Brigade. At the moment of carrying the fence I myself received a wound, which forced me to leave the field, the command of the division devolving upon General Doubleday. On arriving at the foot of the hill, I requested and obtained from General Ricketts a brigade as a support for General Doubleday.

Subordinate reports not having been received by me, I am only able to