The north and south banks of the Potomac river, from the Great Falls, a few miles above Washington, to Harper’s Ferry, were held by the Federal and secession troops respectively. Great care was taken by the Government to defend the north bank, in order to prevent the threatened incursion of the enemy into Maryland, from whence, aided by the disunion sympathizers of that State, they designed to make the long contemplated attack upon the capital. Among the troops stationed on the Potomac, extending from Great Falls to Edwards Ferry, was the division of General Banks; from Edwards Ferry to Conrad’s Ferry, a division under General Stone; while Colonels Lander, Geary, and others held the line thence to Harper’s Ferry. On the south side of the river, two strong positions were held by the enemy—Dranesville and Leesburg. The latter is the terminus of the Loudon and Hampshire railroad, about five miles from the Potomac, and opposite Edwards Ferry. The Southern commanders having determined to abandon their design of crossing the Potomac, had commenced the withdrawal of their troops from various points towards Manassas. General McClellan, anxious to ascertain whether any movement of the forces at Leesburg and Dranesville had been made, directed General McCall, on the 18th, to push a reconnoissance in force in the direction of Dranesville. General McCall penetrated to that town, found that the enemy had evacuated the place, and was informed that Leesburg had also been abandoned. While this reconnoissance was progressing, General McClellan informed General Stone of the fact, and directed him to make careful observations of the movements of the enemy, to ascertain what effect was produced by the expedition of General McCall. He also suggested that a slight demonstration on his own part might be successful in expediting their removal. In obedience to these orders, General Stone, on the 20th, made a feint of crossing the river at Edwards Ferry, while four companies of the Fifteenth Massachusetts were sent to Harrison’s Island, in the Potomac, situated between Edwards and Conrad’s Ferries. At ten o’clock, P. M., Lieutenant Howe, Quartermaster of the Fifteenth Massachusetts, reported that Lieutenant Philbrick had returned to the island from his reconnoissance to Leesburg, and that he had been within one mile of that place, discovering only a small encampment of thirty tents, and without encountering any of the enemy—no pickets being out at any distance from their camp.