NAVY DEPARTMENT, August 20, 1861.

Honorable SIMON CAMERON, Secretary of War:

SIR: The importance of keeping open the navigation of the Potomac is so obvious that no argument is necessary on the subject. So far as is possible this department has and will continue to discharge its duty in this matter by an armed flotilla; but there are one or two points where shore batteries can be made to interrupt communication, and, in view of that danger and recent information, I would most urgently request that immediate measures be taken by the War Department to fortify and intrench Mathias Point. A single regiment, aided by two of our steamers, could heretofore, and perhaps may still, take possession of and secure it. But if more than a regiment is required, it appears to be indispensable that the requisite number should be furnished. Attention on repeated occasions has been called to the particular necessity of holding that place as absolutely essential to the unobstructed navigation of the Potomac. The Navy will at any moment contributed its efforts towards seizing and holding that place, and I apprehend there should not be any delay. Cannot a sufficient force be sent down forthwith to seize and, in connection with such armed vessels as we can order for that purpose, hold Mathias Point, and thus keep open the navigation of the Potomac? I understand that troops will be sent to the Lower Maryland counties, to keep the peace and prevent batteries from being erected on the left bank. This is a timely and wise precaution, by it is equally necessary that we should take possession of Mathias Point. Should the insurgents get possession of that point, it will require a very large force to dispossess them.

I remain, sir, very respectfully, &c.,

GIDEON WELLES.

[Indorsement.]

Respectfully referred to the immediate attention of the Lieutenant-General.

SIMON CAMERON,

Secretary of War.

HEADQUARTERS CORPS OF OBSERVATION,
Poolesville, August 20, 1861.

Major S. WILLIAMS, Headquarters Division of the Potomac:

MAJOR: The condition of this command remains good, and to all appearances the positions of the enemy have not changed opposite us since my letter of yesterday's dare to the General Commanding.

I am still under the impression that there is no very large force in my immediate front, but of course it could be held within one day's march of either of the ferries and yet be out of view.

The river is not deemed fordable here to-day in consequence of the recent rains; but should the rain cease, the water will probably fall in forty-eight hours so as to render three fords passable.

I have received no news from General Banks' command directly; shall send up the river to learn something of his position this evening..

If there is any reasonable chance of an attempt to cross here, I would respectfully ask for at least two more regiments and additional artillery.