Military critics have condemned this act of Lee as one of the gravest blunders of his career. His advance thus far had been resistless, and it was in his power to capture Baltimore, and probably Philadelphia and Washington; but the delay involved in awaiting the return of Jackson gave McClellan, who was a skillful organizer, time to prepare to meet the Confederate invasion. Jackson lost not an hour in capturing Harper's Ferry, the defense of which was so disgraceful that had not Colonel Miles been killed just as the white flag was run up he would have been court-martialed and probably shot. Many suspected him of treason, but the real reason was his cowardice and the fact that he was intoxicated most of the time. Be that as it may, Harper's Ferry surrendered with its garrison of 11,500 men, who were immediately paroled. The Confederates obtained seventy-two cannon, 13,000 small arms, and an immense amount of military stores. Scarcely had the surrender taken place, when Jackson, who had hardly slept for several days and nights, received orders from Lee to join him at once. He started without delay, but he and his men were almost worn out. It is likely that by this time Lee was aware of the mistake he had made when he stopped for several days while his leading assistant went off to capture a post that was of no importance to either side. McCLELLAN'S PURSUIT OF LEE. Leaving a strong garrison to defend Washington, McClellan, at the head of 100,000 troops, set out to follow Lee, who had about 70,000 under his immediate command. The Union leader reached Frederick on the 12th of September, and there a curious piece of good fortune befell him. In the house which had been used as the headquarters of General D.H. Hill was found a copy of an order issued by General Lee, which detailed his projected movements, and contained his instructions to his various leaders. It was priceless information to General McClellan, who made good use of it. Lee manoeuvred to draw McClellan away from Washington and Baltimore, that he might attack them before the Union commander could return to their defense. Lee left Frederick on September 10th, after Jackson had started for Harper's Ferry, and, marching by South Mountain, aimed for Boonsboro'. Stuart and his cavalry remained east of the mountains to watch McClellan, who was advancing with every possible precaution. Lee expected Harper's Ferry would fall on the 13th, but the surrender did not take place until two days later. The Confederate army being divided, McClellan tried to take advantage of the fact, hoping to save Colonel Miles at Harper's Ferry. It did not take Lee long to perceive from the actions of the Union commander that in some way he had learned of his plans.