Attempt the Rebels to Cross the River at Falling Waters—Probable Engagement.

Frederick, Dec 18—A gentleman who arrived from Williamsport late last night reports that Gen. Jackson’s rebel brigade has been largely reinforced, and is supposed to amount to 9,000 men, now at Falling Waters. At 9o’clock p m. 51 wagons belonging to the rebels were backed down to the shore, and a boat from each quietly launched into the river. Two of the boats were of large size, for the transportation of artillery. At this juncture our informant left for General Banks’ headquarters. He thinks the object of Jackson is to cross the river, make a dash on Williamsport, and secure the stores and arms there. He informed Col. Leonard of all that was transpiring, and thinks the latter has taken full precautions to prevent it by removing the stores, etc., to a more secure place, and that he also took steps to concentrate all his available force to prevent the passage of the river.