Howe contemplated an assault, the next morning, upon the American camp, but was deterred by the apparent strength of the lines. These had been built hastily, as General Heath says, of _corn-stalks_, the tops being turned inwards, and the roots with the adhering earth outwards. The British army, strongly reinforced by the arrival of Lord Percy on the 30th, designed attacking the American works on the following day, but a storm delayed their operations, and gave Washington time to withdraw his forces to the heights of New Castle, where he erected strong defences. In the meanwhile Knyphausen had been ordered to move from New Rochelle to Kingsbridge, where he encamped on November 2d, the Americans retiring to Fort Washington on his approach. Howe in person suddenly left White Plains on the night of the 5th for Dobbs's Ferry, to which his army was already moving. "The design of this manœuvre", wrote Washington on the 6th to the President of Congress, "is a matter of much conjecture and speculation, and cannot be accounted for with any degree of certainty." A council of war which met that day evidently inferred that it threatened a movement across or up the Hudson, for it was unanimously agreed immediately to throw a body of troops into New Jersey, and station three thousand at Peekskill to guard the Highlands. Howe really contemplated a far different move—the capture of Fort Washington.
Why Sir William did not again attack Washington, and why he changed his whole plan, is now well understood to be due to the treason of William Demont, the adjutant of Colonel Magaw, in command of Fort Washington. This man, on the 2d of November, undiscovered, passed into the British camp, and placed in the hands of Lord Percy complete plans of the defences of Mount Washington and a statement of their armament and garrisons. This detailed information was immediately sent, with its author, to Howe, and must have reached him a day or two before his sudden departure from White Plains. The conclusive evidence of this treason is furnished by the culprit himself in his letter,[722] dated London, January 16, 1792, to the Rev. Dr. Peters, of the Church of England, which was first published by Mr. E. F. DeLancey, in the _Magazine of American History_ (Feb., 1877).