WARRENTON JUNCTION, August 27,[1862]-4 p.m.

General BURNSIDE, Falmouth:

I send you the last order from General Pope, which indicate the future as well as the present. Wagons are rolling along rapidly to the rear, as if a mighty power was propelling them. I see to cause for alarm, though I think this order my cause it. McDowell moves to Gainesville, where Sigel now is. the latter got to Buckland Bridge in time to put out the fire and kick the enemy, who is pursuing his route unmolested to the Shenandoah, or Loudoun County. The forces are Longstreet's, A. P. Hill's, Jackson's, Whiting's, Ewell's, and Anderson's (late Huger's) divisions. Longstreet is said by a deserter to be very strong. They have much artillery and long wagon trains. The raid on the railroad was near Cedar Run, and made by a regiment of infantry, two squadrons of cavalry, and a section of artillery. The place was guarded by nearly three regiments of infantry, and some cavalry. They routed the guard, captured a train and many men, destroyed the bridge, and retired leisurely down the road toward Manassas. It can be easily repaired. No troops are coming up, except new troops, that I can hear of. Sturgis is here with two regiments. Four were cut off by the raid. The positions of the troops are given in the order. No enemy in our original front. A letter of General Lee, seized when Stuart's assistant adjutant-general was taken, directs Stuart to leaven a squadron only to watch in front of Hanover Junction, &c. Everything has moved up north. I find a vast difference between these troops and ours. But I suppose they were new, as they to-day burned their clothes, &c., when there was not the least cause. I hear that they are much demoralized, and needed some good troops to give them heart, and, I think, head. We are working now to get behind Bull Run, and, I presume, will be there in a few days, if strategy don't use us up. The strategy is magnificent, and tactics in the inverse proportion. I would like some of my ambulances; I would like also to be ordered to return to Fredericksburg and to push toward Hanover, or, with a larger force, to strike at Orange Court-House. I wish Sumner was at Washington, and up near the Monocacy with good batteries. I do not doubt the enemy have large amounts of supplies provided for them, and I believe they have a contempt for this Army of Virginia. I wish myself away from it, with all our old Army of the Potomac, and so do our companions. I was informed to-day, by the best authority, that, in opposition to General Pope's views, this army was pushed out to save the Army of the Potomac, an army that could take the best care of itself. Pope says he long since wanted to go behind the Occoquan. I am in great need of ambulances, and the officers need medicines, which, for want of transportation, were left behind. I hear many of the sick of my corps are in house on the road, very sick. I think there is no fear of an enemy crossing the Rappahannock. The cavalry are all in the advance of the rebel army. At Kelly's and Barnett's Fords much property was left, in consequence of the wagons going down for grain, &c. If you can push up the grain to-night, please do so, direct to this place. There is no grain here to-day, or anywhere, and this army is wretchedly supplied in that line. Pope says he never could get enough.

F. J. PORTER.

Most of this is private. But if you can get me away, please do so. Make what use of this you choose, so it does good. Don't left the alarm