From The Shenandoah

Baltïmóke, Aug. 29. A special to tbc army headquarters, Middle Military División, in the field, 29th, gaya the long-lnokcd fov advancc of our troops has 'at last cofnmenoed. Yesterday morning, at 4 o'clock, our cavalry startod out and commcuccd driving the enemy's rcar guard. At 5 o'cloek the itifautry broke camp, and at; 7 oYlock liad taken a position in a line of march which was in three parallel columns Wc luid not proeeeded fur, whon we beeame sktrefiètf, tliat the niain force of tho enemy liad retired frora our immediale front, and retroated to somc position up tlio valley. A reconnoissanee made in front of General Oook's lines on the left of our positiou. Saturday, developed the fact tliat tiie main forcea of the enemy had left our irainediate fi-ont, but iu wliat direction tli-ay had moved, we liad no reliable information.'and even. up to tho time of writiog this dispateh, the whercabouts of Eariy's main force is a mere speeulation. Movements, however, nt present are on foot which throw some light on the subject. Our cavalry had some heavy skirmislung yesterday in the vicinity of Charlostown, with a portion of Early's rear guard, iu whieh two officers of the lst United States Cuvalry worc killed. We captured souie twenty prisoners, who state tliat tho main force of the enemy moved oli' three daysago, and went in the direction of Martiusburg. Statements of prisoners have to be tf.kcn with a great deal of caution.