To maintain the command of the water as far as was practicable, congress had ordered impediments to be sunk in many of the rivers of common use, so as to obstruct the passage up them, and had constructed frigates, and other smaller vessels, to be employed above those impediments or elsewhere, as the occasion might require. Several of them had been commenced above Philadelphia, but were not completed when the British obtained the command of the river. General Washington then became apprehensive for their safety, and repeatedly expressed his desire that they should be sunk in such a manner as to be weighed with difficulty, should any attempt be made to raise them. The persons, however, who were entrusted by congress with this business, supposed it would be equally secure to put plugs in their bottoms, which might be drawn out on the approach of danger. Against these vessels, and some stores collected at Bordentown, an expedition was planned which ended in their total destruction. General Dickenson was in the neighbourhood, but his force was too small to interrupt the execution of the design; and General Maxwell, who had been ordered to his assistance, was retarded in his march by a heavy rain, which did not obstruct the movement of the British, who passed up the river in vessels. [Sidenote: May 18.] To cover the country more effectually on the north of the Schuylkill, to form an advance guard for the security of the main army, and to be in readiness to annoy the rear of the enemy, should he evacuate Philadelphia, an event believed to be in contemplation, General Washington detached the Marquis de Lafayette, with more than two thousand choice troops, to take post near the lines. As this corps formed a very valuable part of the army, the Commander-in-chief recommended in his instructions to General Lafayette the utmost attention to its safety; and, particularly, to avoid any permanent station, as a long continuance in one position would facilitate the execution of measures which might be concerted against him. [Sidenote: Attempt on Lafayette at Barren Hill.] The Marquis crossed the Schuylkill and took post near Barren Hill church, eight or ten miles in front of the army. Immediate notice[2] of his arrival was given to Sir William Howe, who reconnoitred his position, and formed a plan to surprise and cut him off.