The defence being unexpectedly obstinate, the assailants brought up their ships as far as the obstructions in the river permitted, and added their fire to that of the batteries, which was the more fatal as the cover for the troops had been greatly impaired. The brave garrison, however, still maintained their ground with unshaken firmness. In the midst of this stubborn conflict, the Vigilant and a sloop of war were brought up the inner channel, between Mud and Province Islands, which had, unobserved by the besieged, been deepened by the current in consequence of the obstructions in the main channel; and, taking a station within one hundred yards of the works, not only kept up a destructive cannonade, but threw hand grenades into them; while the musketeers from the round top of the Vigilant killed every man that appeared on the platform. Major Thayer applied to the Commodore to remove these vessels, and he ordered six galleys on the service; but, after reconnoitring their situation, the galleys returned without attempting any thing. Their report was that these ships were so covered by the batteries on Province Island as to be unassailable. [Sidenote: Fort Mifflin evacuated and possession taken by the British.] {November 16.} It was now apparent to all that the fort could be no longer defended. The works were in ruins. The position of the Vigilant rendered any farther continuance on the island a prodigal and useless waste of human life; and on the 16th, about 11 at night, the garrison was withdrawn.[72] [Footnote 72: In stating the defence of Mud Island, the author has availed himself of the journal of Major Fleury.] A second attempt was made to drive the vessels from their stations with a determination, should it succeed, to repossess the island; but the galleys effected nothing; and a detachment from Province Island soon occupied the ground which had been abandoned. {November 17.} The day after receiving intelligence of the evacuation of fort Mifflin, General Washington deputed Generals De Kalb, and Knox, to confer with General Varnum and the officers at fort Mercer on the practicability of continuing to defend the obstructions in the channel, to report thereon, and to state the force which would be necessary for that purpose. Their report was in favour of continuing the defence. A council of the navy officers had already been called by the Commodore in pursuance of a request of the Commander-in-chief made before the evacuation had taken place, who were unanimously of opinion that it would be impracticable for the fleet, after the loss of the island, to maintain its station, or to assist in preventing the chevaux-de-frise from being weighed by the ships of the enemy.