DUMFRIES, VA., December 13, 1862.

Major-General SLOCUM,

Twelfth Corps, Fairfax Court-House, Va.:

Your dispatch received. Please inform me by letter what is the effective strength of your corps in infantry, cavalry, and artillery. General Burnside has crossed the Rappahannock, and wishes that we advance as quickly as possible. A probably severe battle has being going on the whole day. Firing can be heard here. It is, therefore, absolutely necessary that you march to-morrow evening, if you have to leave your cavalry one day behind. In traveling, myself, the road by Wolf Run Shoals to Dumfries, I find it extremely bad, not only on account of the weather, but also on account of being worked out by the different divisions that have taken this road. But I learn also that the road from Dumfries to Brentsville is not better. It is, therefore, necessary that, in following us by Wolf Run Shoals, you should send a party of about 300 pioneers in advance immediately, to repair the roads, and to be stationed at the places which have been repaired until your trains have passed them. Unless you do this you cannot well get along with your artillery and trains. Your cavalry should follow your column in the rear, so as not to interfere with your infantry and make the road more impassable. From Fairfax Station you may send a part of your wagons, under guard, to Occoquan City and the Telegraph road to Dumfries, as there is a ferry at Occoquan which can bring over 12 wagons in one hour. In marching from Wolf Run Shoals to Dumfries you have not much to fear on your right, as my cavalry is stationed between Brentsville and Dumfries; but send out your patrols and flankers. Yesterday 500 of the enemy's cavalry made an attack on Dumfries, took some sutlers' wagons, a telegraph operator, and 15 men of our cavalry. They fled as soon as we approached the town.

F. SIGEL,

Major-General.

WAR DEPARTMENT,

Washington, December 13, 1862.

Major-General SLOCUM,

Fairfax Court-House, Va.:

Your command should move forward rapidly, so as to get into supporting distance of General Burnside, who is engaged in a severe battle at Fredericksburg.

H. W. HALLECK,

General-in-Chief.

FAIRFAX COURT-HOUSE, VA., December 13, 1862.

General H. W. HALLECK,

General-in-Chief:

One division of my command will encamp at this place to-night; the others at Chantilly. Telegraphic communication with General Sigel is cut off. I shall move on toward Dumfries to-morrow, via Wolf Run Shoals, unless otherwise ordered. Our march from Harper's Ferry has been accomplished without the loss of any property, and the troops are in good condition and fine spirits.

H. W. SLOCUM,

Major-General, Commanding.