The movements of the Federal army of the Potomac
[Special Dispatch to the Richmond Dispatch.] Army of Northern Virginia, July 19.
--The enemy in force is reliably reported to have crossed into Loudoun county, and are making demonstrations at Snicker's Ferry, on the Shenandoah, and at Snicker's Gap.
Yesterday several hundred Yankee cavalry advanced from the river to within four miles of Martinsburg, and were driven back by a regiment of our cavalry.