On the morning of the 13th instant we rejoined our brigade, anticipating an attack. Riflepits were thrown up and due preparation made. The enemy are strongly fortified between St. James’ college and Williamsport, some two miles distant, and on the 14th he was reported as falling back across the Potomac. Heavy cannonading is heard towards the river, and the First division of General Williams advances in reconnoissance, the Second acting as support. A dispatch states that Lee’s army, in full retreat, began crossing the river yesterday, continued through the night and to-day. Our troops are hotly engaged with his rear guard, and we remain in position until the morning of the 18th, when we move briskly forward in pursuit of the fleeing chivalry, who were now across the Potomac in Virginia. We marched via Sharpsburg and Antietam. While passing through the woods three men were shot by Mosby’s cavalry. A march of sixteen miles and we halt within three miles of Harper’s Ferry. At Sandy Hook we remain until the morning of the 19th instant, when we move forward via Harper’s Ferry, to Hillsboro, Virginia. On the following morning we are early on the move, advancing as far as Snickersville, near the gap of that name, where pickets are thrown out and we bivouac for the night. We remain here for two days, during which time we muster and pass in review. Colonel W. T. Fitch recently promoted, and who has been absent on leave since March 28th, joined us here, and assumed command. On the morning of the 23d we again moved forward, passing through Upperville and on to Ashby’s gap, where, at a late hour, we go into camp. Having marched thirty miles since morning sleep was sweet that night, with mother earth for a bed, and only heaven’s blue canopy for a cover. At 3 o’clock on the morning of the 24th we march rapidly forward through the villages of Markham and Linden, halting for dinner at Manassas gap, then forward, changing direction by column to the south-southeast, in the direction of White Plains. Marching sixteen miles we halted for the night. The next day we marched to White Plains, and the next by the way of Thoroughfare gap, Haymarket, and Greenwich, the latter a pretty little village, settled by English people, for whose protection guards were stationed, as they were in fact in many instances for the protection of rebel property. After a brief halt we marched forward via Catlett’s station and Warrenton, where we joined the First and Eleventh corps of our army and encamped for the night, then forward again to Kelley’s Ford, on the Rappahannock, when, after a short visit, the brigade took cars for Alexandria, Virginia, under orders for New York city, to quell the memorable draft riots induced by the Southern sympathizers and copperheads.