The headquarters of Gen. McClellan were two or three miles north of Knoxville, a little village on the Potomac, about three miles below Harper's Ferry. The day that we were there, the General was absent on his way to meet Mrs. McClellan, and though the telegraph wires ran to headquarters, nothing was there known of the foray Stuart had begun early that morning from Hancock, in the rear of our forces; not till evening, and until his arrival at Chambersburg did the news arrive. If the telegraph wires had been laid, or the signal corps so stationed as to have given warning of the inception of this movement, these bold rebels could not have advanced so far, but would have been compelled to retreat as they came. Between the General's headquarters and the river were the famous sixth cavalry of regulars and some batteries of artillery. He had no guard in the direction of Pennsylvania toward the northeast, where Stuart's cavalry passed on their way to the Potomac. The camp itself was not well placed, and was soon changed. In going from it we rode through a most beautiful country by the side of an officer of the sixth cavalry, and listened to his enthusiastic account of scouting in front of our lines, in the footsteps of the retreating enemy, over the very roads we were travelling safely and without concern; and yet we were not many miles from the foe, and within reach of the marvellous flight of the minie ball, which some lurking rifleman might aim from the other side of the Potomac. These cavalry soldiers and horses have had a terribly hard time of it. The horses of the sixth were more broken down and thinner than in the artillery or baggage trains. Two squadrons had lately been part of the force sent on a reconnoisance to Leesburg; and upon the return of our troops it had been the duty of our companion, then in command, to bring up the rear and drive in the infantry stragglers. Some two hundred had fallen out of the ranks from mere exhaustion. To leave any of these soldiers behind would be giving them up as prisoners, and affording the enemy the opportunity of obtaining information which it was of the utmost importance for the safety of the expedition to keep back. The troopers had therefore to drive them on with their swords--not a pleasant duty, when the poor fellows were faint and used up by fatigue--still it must be done. This service creates quite a dislike between the two arms. The infantry man hates the horseman, and the cavalry man despises the foot soldier. At this time straggling was quite prevalent: we saw on byroads many who had left the ranks, almost invariably having thrown away their arms, and subsisting on plunder. The cavalry were scouring the roads for them, and were bringing them in as prisoners for punishment. This sixth cavalry, like all the old regiments which had been through the Peninsular campaign and the disastrous retreat under Pope, was frightfully reduced in numbers: