On the night of the battle of South Mountain, orders were given to the corps commanders to press forward the pickets at early dawn. This advance revealed the fact that the enemy had left his positions, and an immediate pursuit was ordered; the cavalry, under Gen. Pleasonton, and the three corps under Gens. Sumner, Hooker, and Mansfield, (the latter of whom had arrived that morning and assumed command of the 12th, Williams's corps,) by the national turnpike and Boonsboro'; the corps of Gens. Burnside and Porter (the latter command at that time consisting of but one weak division, Sykes's) by the old Sharpsburg road, and Gen. Franklin to move into Pleasant valley, occupy Rohrersville by a detachment, and endeavor to relieve Harper's Ferry.

Gens. Burnside and Porter, upon reaching the road from Boonsboro' to Rohrersville to re-enforce Franklin, or to move on Sharpsburg, according to circumstances.

Franklin moved towards Brownsville and found there a force of the enemy, much superior in numbers to his own, drawn up in a strong position to receive him. At this time the cessation of firing at Harper's Ferry indicated the surrender of that place.

The cavalry overtook the enemy's cavalry in Boonsboro', made a daring charge, killing and wounding a number, and capturing 250 prisoners and two guns.

Gen. Richardson's division of the 2d corps pressing the rear guard of the enemy with vigor, passed Boonsboro' and Keedysville, and came upon the main body of the enemy, occupying in large force a strong position a few miles beyond the latter place.

It had been hoped to engage the enemy during the 15th. Accordingly, instructions were given that if the enemy were overtaken on the march they should be attacked at once; if found in heavy force and in

position, the corps in advance should be placed in position for attack, and await my arrival. On reaching the advanced position of our troops, I found but two divisions, Richardson's and Sykes's, in position; the other troops were halted in the road; the head of the column some distance in rear of Richardson.

The enemy occupied a strong position on the heights, on the west side of Antietam creek, displaying a large force of infantry and cavalry, with numerous batteries of artillery, which opened on our columns as they appeared in sight on the Keedysville road and Sharpsburg turnpike, which fire was returned by Capt. Tidball's light battery, 2d United States artillery, and Pettit's battery, 1st New York artil. lery.

'T'he division of Gen. Richardson, following close on the heels of the retreating foe, halted and deployed near Antietam river, on the right of the Sharpsburg road. Gen. Sykes, leading on the division of regulars on the old Sharpsburg road, came up and deployed to the left of Gen. Richardson, on the left of the road.

Antietam creek, in this vicinity, is crossed by four stone bridges-the upper one on the Keedysville and Williamsport road; the second on the Keedysville and Sharpsburg turnpike, some two and a half miles below; the third about a mile below the second, on the Rohrersville and Sharpsburg road; and the fourth near the mouth of Antietam creek, on the road leading from Harper's Ferry to Sharpsburg, some three miles below the third. The stream is sluggish, with few and difficult fords. After a rapid examination of the position, I found that it was too late to attack that day, and at once directed the placing of the batteries in position in the centre, and indicated the bivouacs for the different corps. massing them near and on both sides of the Sharpsburg turnpike. The corps were not all in their positions until the next morning after sunrise.

On the morning of the 16th, it was discovered that the enemy had changed the position of his batteries. The masses of his troops, however, were still concealed behind the opposite heights. Their left and centre were upon and in front of the Sharpsburg and Hagerstown turnpike, hidden by woods and irregularities of the ground; their extreme left resting upon a wooded eminence near the cross-roads to the north of J. Miller's farm; their left resting upon the Potomac. Their line extended south, the right resting upon the hills to the south of Sharpsburg, near Shaveley's farm.

The bridge over the Antietam, described as No. 3, near this point, was strongly covered by riflemen protected by rifle-pits,

stone fences. &c., and enfiladed by artillery. The ground in front of this line consisted of undulating hills, their crests in turn commanded by others in their rear. On all favorable points the enemy's artillery was posted and their reserves hidden from view by the hills, on which their line of battle was formed, could manœuvre unobserved by our army, and from the shortness of their line could rapidly reenforce any point threatened by our attack. Their position, stretching across the angle formed by the Potomac and Antietam, their flanks and rear protected by these streams, was one of the strongest to be found in this region of country, which is well adapted to defensive warfare.

On the right, near Keedysville, on both sides of the Sharpsburg turnpike, were Sumner's and Hooker's corps. In advance, on the right of the turnpike and near the Antietam river, Gen. Richardson's division of Gen. Sumner's corps was posied. Gen. Sykes's division of Gen. Porter's corps was on the left of the turnpike and in line with Gen. Richardson, protecting the bridge No. 2, over the Antietam. The left of the line, opposite to and some distance from bridge No. 3, was occupied by Gen. Burnside's corps.

Before giving Gen. Hooker his orders to make the movement which will presently be described, I rode to the left of the line to satisfy myself that the troops were properly posted there to secure our left flank from any attack made along the left bank of the Antietam, as well as to enable us to carry bridge No. 3.

I found it necessary to make considerable changes in the position of Gen. Burnside's corps, and directed him to advance to a strong position in the immediate vicinity of the bridge, and to reconnoitre the approaches to the bridge carefully. In front of Gen. Sumner's and Hooker's corps, near Keedysville, and on the ridge of the first line of hills overlooking the Antietam, and between the turnpike and Fry's house on the right of the road, were placed Capt. Taft's, Langner's, Von Kleizer's and Lieut. Weaver's batteries of twenty-pounder Parrott guns. On the crest of the hill in the rear and right of bridge No. 8, Capt. Weed's three-inch and Lieut. Benjamin's twenty-pounder batteries. Gen. Franklin's corps and Gen. Couch's division held a position in Pleasant valley in front of Brownsville, with a strong force of the enemy in their front. Gen. Morell's division of Porter's corps was en route from Boonsboro', and Gen. Humphrey's_division of new troops en route from Frederick, Maryland. About daylight on the 16th the enemy opened a heavy fire of artillery on our guns in posi

tion, which was promptly returned; their fire was silenced for the time, but was frequently renewed during the day. In the heavy fire of the morning, Maj. Arndt, commanding first battalion first New York artillery, was mortally wounded while directing the operations of his batteries.

It was afternoon before I could move the troops to their positions for attack, being compelled to spend the morning in reconnoitring the new position taken up by the enemy, examining the ground, finding fords, clearing the approaches, and hurrying up the ammunition and supply trains, which had been delayed by the rapid march of the troops over the few practicable approaches from Frederick. These had been crowded by the masses of infantry, cavalry and artillery pressing on with the hope of overtaking the enemy before he could form to resist an attack. Many of the troops were out of rations on the previous day, and a good deal of their ammunition had been expended in the severe action of the 14th.

My plan for the impending general engagement was to attack the enemy's left with the corps of Hooker and Mansfield, supported by Sumner's, and if necessary by Franklin's; and, as soon as matters looked favorably there, to move the corps of Burnside against the enemy's extreme right, upon the ridge running to the south and rear of Sharpsburg, and having carried their position, to press along the crest towards our right; and whenever either of these flank movements should be successful, to advance our centre with all the forces then disposable.

About 2 P. M., Gen. Hooker, with his corps, consisting of Gen. Rickett's, Meade's and Doubleday's divisions, was ordered to cross the Antietam at a ford, and at bridge No. 1, a short distance above, to attack, and, if possible, turn the enemy's left. Gen. Sumner was ordered to cross the corps of Gen. Mansfield (the 12th) during the night, and hold his own (the 2d) corps ready to cross early the next morning. On reaching the vicinity of the enemy's left, a sharp contest commenced with the Pennsylvania reserves, the advance of Gen. Hooker's corps, near the house of D. Miller. The enemy were driven from the strip of woods where he was first met. The firing lasted until after dark, when Gen. Hooker's corps rested on their arms on ground won from the enemy.

During the night Gen. Mansfield's corps, consisting of Gen. Williams's and Green's divisions, crossed the Antietam at the same ford and bridge that Gen. Hooker's troops had passed, and bivouacked on the farm of J. Poffenberger, about a mile in rear of Gen. Hooker's position. At daylight on

the 17th the action was commenced by the skirmishers of the Pennsylvania reserves. The whole of Gen. Hooker's corps was Boon engaged, and drove the enemy from the open field in front of the first line of woods into a second line of woods beyond which runs to the eastward of and nearly parallel to the Sharpsburg and Hagerstown turnpike.

This contest was obstinate, and as the troops advanced the opposition became more determined and the number of the enemy greater. Gen. Hooker then ordered up the corps of Gen. Mansfield, which mored promptly toward the scene of action.

The first division, Gen. Williams's, was deployed to the right on approaching the enemy; Gen. Crawford's brigade on the right, its right resting on the Hagerstown turnpike; on his left Gen. Gordon's brigade. The second division, Gen. Green's, joining the left of Gordon's extended as far as the burnt buildings to the north and east of the white church on the turnpike. During the deployment, that gallant veteran Gen. Mansfield fell mortally wounded, while examining the ground in front of his troops. Gen. Hartsuff, of Hooker's corps, was severely wounded, while bravely pressing forward his troops, and was taken from the field.

The command of the twelfth corps fell upon Gen. Williams. Five regiments of first division of this corps were new troops. One brigade of the second division was sent to support Gen. Doubleday.

The one hundred and twenty-fourth Pennsylvania volunteers were pushed across the turnpike into the woods beyond J. Miller's house, with orders to hold the position as long as possible.

The line of battle of this corps was formed, and it became engaged about seven A. M., the attack being opened by Knapp's (Pennsylvania,) Cothran's (New York,) and Hampton's (Pittsburg) batteries. To meet this attack the enemy had pushed a strong column of troops into the open fields in front of the turnpike, while he occupied the woods on the west of the turnpike in strong force. The woods (as was found by subsequent observation) were traversed by outcropping ledges of rock. Several hundred yards to the right and rear was a hill which commanded the debouche of the woods, and in the fields between was a long line of stone fences, continued by breast works of rails, which covered the enemy's infantry from our musketry. The same woods formed a screen behind which his movements were concealed, and his batteries on the hill and the rifle works covered from the fire of our artillery in front.

For about two hours the battle raged with varied success, the enemy endeavoring to drive our troops into the second line of wood, and ours in turn to get possession of the line in front.

Our troops ultimately succeeded in forcing the enemy back into the woods near the turnpike, Gen. Green with his two brigades crossing into the woods to the left of the Dunbar church. During this conflict Gen. Crawford, commanding first division after Gen. Williams took command of the corps, was wounded and left the field.

Gen. Green being much exposed and applying for re-enforcements, the thirteenth New Jersey, twenty-seventh Indiana, and the third Maryland were sent to his support with a section of Knapp's battery.

At about nine o'clock A. M. Gen. Sedgwick's division of Gen. Sumner's corps arrived. Crossing the ford previously mentioned, this division marched in three columns to the support of the attack on the enemy's left. On nearing the scene of action the columns were halted, faced to the front, and established by Gen. Sumner in three parallel lines by brigade, facing toward the south and west; Gen. Gorman's brigade in front, Gen. Dana's second, and Gen. Howard's third, with a distance between the lines of some seventy paces. The division was then put in motion and moved upon the field of battle, under fire from the enemy's concealed batteries on the hill beyond the roads. Passing diagonally to the front across the open space and to the front of the first division of Gen. Williams's corps, this latter division withdrew.

Entering the woods on the west of the turnpike, and driving the enemy before them, the first line was met with heavy fire of musketry and shell from the enemy's breastworks and the batteries on the hill commanding the exit from the woods: meantime a heavy column of the enemy had succeeded in crowding back the troops of Gen. Green's division, and appeared in rear of the left of Sedgwick's division. By command of Gen. Sumner, Gen. Howard faced the third line to the rear preparatory to a change of front to meet the column advancing on the left; but this line now suffering from a destructive fire both in front and on its left, which it was unable to return, gave way towards the right and rear in considerable confusion, and was soon followed by the first and second lines.

Gen. Gorman's brigade, and one regi ment of Gen. Dana's, soon rallied and checked the advance of the enemy on the right. The second and third lines now

on the field, the enemy directed a fire of artillery against it, but owing to the irregularities of the ground did but little damage. Meagher's brigade advancing steadily soon became engaged with the enemy posted to the left and in front of Roufette's house. It continued to advance under a heavy fire nearly to the crest of the hill overlooking Piper's house, the enemy being posted in a continuation of the sunken road and cornfield before referred to. Here the brave Irish brigade opened apon the enemy a terrific musketry fire.

All of Gen. Sumner's corps was now engaged: Gen. Sedgwick on the right; Gen. French in the centre, and Gen. Richardson on the left. The Irish brigade sustained its well-earned reputation. After suffering terribly in officers and men, and strewing the ground with their enemies as they drove them back, their ammunition nearly expended, and their commander, Gen. Meagher, disabled by the fall of his horse shot under him, this brigade was ordered to give place to Gen. Caldwell's brigade, which advanced to a short distance in its rear. The lines were passed by the Irish brigade breaking by company to the rear, and Gen. Caldwell's by company to the front as steadily as on drill. Col. Brooks's brigade now became the second line.

The ground over which Gens. Richardson's and French's divisions were fighting was very irregular, intersected by numerous ravines, hills covered with growing corn, enclosed by stone walls, behind which the enemy could advance unobserved upon any exposed point of our lines. Taking advantage of this, the enemy attempted to gain the right of Richardson's position in a cornfield near Roulette's house, where the division had become separated froin that of Gen. French's. A change of front by the 52d New York and 2d Delaware volunteers, of Col. Brooks's brigade, under Col. Frank, and the attack made by the 53d Pennsylvania volunteers, sent further to the right by Col. Brooks to close this gap in the line, and the movement of the 132d Pennsylvania and 7th Virginia volunteers of Gen. French's division, before referred to, drove the enemy from the cornfield and restored the line.

The brigade of Gen. Caldwell, with determined gallantry, "pushed the enemy back opposite the left and centre of this division, but sheltered in the sunken road, they still held our forces on the right of Caldwell in check. Col. Barlow command ing the 61st and 64th New York regiments of Caldwell's brigade, seeing a favorable opportunity, advanced the regiments on the left, taking the line in the sunken road in flank, and compelling them to surrender,

capturing over three hundred prisoners and three stands of colars.

The whole of the brigade, with the 57th and 66th New York regiments of Col. Brooks's brigade, who had moved these regiments into the first line, now advanced with gallantry, driving the enemy before them in confusion into the cornfield beyond the sunken road. The left of the division was now well advanced, when the enemy, concealed by an intervening ridge en deavored to turn its left and rear.

Col. Cross, 5th New Hampshire, by a change of front to the left and rear, brought his regiment facing the advancing line. Here a spirited contest arose to gain a commanding height, the two opposing forces moving parallel to each other, giving and receiving fire. The 5th gained the advantage, faced to the right and delivered its volley. The enemy staggered, but rallied and advanced desperately at a charge. Being re-enforced by the 81st Pennsylvania, these regiments met the advance by a counter charge. The enemy fled leaving many killed, wounded, and prisoners, and the colors of the 4th North Carolina, in our hands.

Another column of the enemy, ad vancing under shelter of a stone wall and cornfield, pressed down on the right of the division; but Col. Barlow again advanced the 61st and 64th New York against these troops, and with the attack of Kimball's brigade on the right, drove them from this position.

Our troops on the left of this part of the line having driven the enemy far back, they, with re-enforced numbers, made a determined attack directly in front. To meet this, Col. Barlow brought his two regiments to their position in line, and drove the enemy through the cornfield into the orchard beyond, under a heavy fire of musketry, and a fire of canister from two pieces of artillery in the orchard, and a battery further to the right, throwing shell and case shot. This advance gave us possession of Piper's house, the strong point contended for by the enemy at this part of the line, it being a defensible building several hundred yards in advance of the sunken road. The musketry fire at this point of the line now ceased. Holding Piper's house, Gen. Richardson withdrew the line a little way to the crest of a hill, a more advantageous position. Up to this time the division was without artillery, and in the new position suffered severely from artillery fire which could not be replied to. A section of Robertson's horse battery, commanded by Lieut. Vincent, 2d artillery, now arrived on the ground and did excellent service. Subsequently a battery of brass guns, commanded by Capt.