HEADQUARTERS ARMY OF THE POTOMAC,
December 30, 1862.

Brigadier-General AVERELL, Commanding Cavalry Brigade:

SIR: The general commanding directs that, owing to information received from Washington, you postpone for the present your movement, as agreed upon before your departure. He requests that you communicate with the infantry force and direct in his name that they return leisurely to their camps. With your available cavalry force, the general wishes you to intercept, if possible, the force of the enemy's cavalry that recently attacked Dumfries, and made a raid upon the Orange and Alexandria Railroad to the east of Fairfax Station. A dispatch from General Heintzelman, of 2.20 p.m. to-day, states that "our cavalry in pursuit of the rebels have not been heard from since they were at Chantilly. The enemy retreated toward Aldie."

It is not desirable thay you should go much beyond Warrenton, excepting with scouts, using, however, your own judgment. You will communicate with these headquarters by way of Catlett's Station and Hartwood.

JNumbers G. PARKE,

WASHINGTON, December 30, 1862.

Honorable E. M. STANTON, Secretary of War:

SIR: I herewith present you the report of the Commission ordered by you to examine the Defenses of Washington, and report to you as to their efficiency, &C.

It will be seen that the Commission approve generally of the lines established and of the works, and that they attach very great importance to them; that they attach very great importance to them; that they recommend some a additions to or modifications of the existing works; some new works (five or six) to strengthen certain parts of the line, and that they purpose to add a new feature to the defensive system, by the construction of works to defend the river from maritime attack. Their reasons are given in full, and it is not necessary for me to dwell upon them in this place.

The amount expended upon the system up to time when I relinquished the charge last spring to take the field with the Army of the Potomac was about $550,000. This applied to the construction of upward of fifty forts and a number of batteries. Some of these works were of large dimensions, and many had, besides the usual magazine, extensive bomb-proofs, for the protection of the garrisons.

Notwithstanding the number of works built, the defensive system was in some parts still very weak, and everywhere there was need (as I stated in a report to the Chief Engineer U. S. Army a year ago) of auxiliary works, more efficient armament, &c., and I also stated that there were important gaps in the line which should be filled.

When the Army of the Potomac retired from the James River, I was ordered to assume the command of the works and troops of Washington, and there was apprehension felt (as the result proved, rightly) for the safety of Washington.

Of course, it was my duty, both as engineer and commanding officer, to use the time and means disposable to increase the strength of the defenses. The norther side of the city, between the Potomac and Eastern Branch, which had been little exposed to attack the summer before, was, in August and September of this year, the most likely to be assailed and from the Potomac to the Seventh street road it was exceedingly weak.