[In his report to Major Barnard, Capt. D. P. Woodbury, of the corps of Engineers, says: "It is not for me to give a history of the battle. The Enemy was driven on our left, from cover to cover, a mile and a half. Our position for renewing the action the next morning was excellent; whence, then, our failure? It will not be out of place, I hope, for me to give my own opinion of the cause of this failure. An old soldier feels safe in the ranks, unsafe out of the ranks, and the greater the danger the more pertinaciously he clings to his place. The volunteer of three months never attains this instinct of discipline. Under danger, and even under mere excitement, he flies away from his ranks, and looks for safety in dispersion. At four o'clock in the afternoon of the 21st, there were more than twelve thousand volunteers on the battle-field of Bull Run, who had entirely lost their regimental organizations. They could no longer be handled as troops, for the officers and men were not together. Men and officers mingled together promiscuously; and it is worthy of remark that this disorganization did not result from defeat or fear, for up to four o'clock we had been uniformly successful. The instinct of discipline, which keeps every man in his place, had not been acquired. We cannot suppose that the troops of the Enemy had attained a higher degree of discipline than our own, but they acted on the defensive, and were not equally exposed to disorganization."] While the divisions of Hunter and Heintzelman, which came down in the morning across Sudley Ford, are now, with one brigade (Sherman's) of Tyler's Division, retiring again, in this disordered condition, by that ford; two other brigades of Tyler's Division, viz., that of Schenck --which, at 4 o'clock, was just in the act of advancing upon, and across, the Stone Bridge, to join in the Union attack, and of Keyes, which was, at the same time, just succeeding in its effort to turn the right flank of the Enemy's third new line,--are withdrawing from the field, across Bull Run stream, by the Warrenton Pike, and other roads leading them directly toward Centreville. The brigades of both Keyes and Schenck are retiring in good order; that of Keyes, at "an ordinary pace," following close after McDowell, who, with his staff, has ridden across the battlefield and Bull Run; while part of that of Schenck, united with the 2nd Maine (of Keyes' Brigade) and Ayres's Battery, "promptly and effectively" repulses a charge of the Enemy's Cavalry, and covers the rear of Tyler's Division. Both of these brigades reach Centreville, hungry and weary, but otherwise, for the most part, in good shape.