22 feet water can pass up the Potomac within a hundred yards of the Arsenal. As far as the light-house on Blackistone Island, some 30 miles, there is abundant water for any ship. About 5 miles higher up is the first obstacle-the Kettle Bottoms. The channel passes among these shoals for 5 miles, and the pilot says 24 feet can be had, which I doubt. The narrowest part about 300 yards wide. From this the channel continued good until just below Aquia, where it shoals, so that 23 feet is considered the best water at common high tide. Having passed this, the water deepens passing the batteries and shoals about Mattawoman Creek, where the dept at common high tide is 22 feet. This obstruction is less than a mile in extent, after which the channel deepens served feet; though it narrows; it runs nearly to the Arsenal, and perhaps some 3 miles from the Capitol.

The actual blocking of the river is only to be resorted to when the exigency arises, the means being at hand. There are three points where it can be done-the Kettle Bottoms, below Smith's Point, and at Mattawoman. I would advise that some heavy ordnance be got ready for placing at the Arsenal, at Giesborough Point, and at Buzzard Point. Fort Washington should also have suitable cannon. I have telegraphed to the flotilla for some steamers to tow down the blocking vessels as soon as General Meigs hat them ready. It happens unfortunately that only two good steamers belonging to the yard are at Fortress Monroe.

JNO. A. DAHLGREN,

Commandant,

MARCH 9, 1862-3.40 p. m.

General McCLELLAN, Commander:

I am making arrangements to place an 11-inch gun and some 10-inch mortars on Giesborough Point, which will command at short range the nearest point that a vessel drawing 22 feet can approach the Capitol. The channel passes within 50 yards of this position.

As I have but a handful of men, it might be convenient to have some assistance from the neighboring regiments. If so, please authorize it.

JNO. A. DAHLGREN.

WAR DEPARTMENT, March 9, 1862-5.30 p. m.

Captain DAHLGREN:

The steamer Sophia will leave G-street wharf in ten minutes, having in two eight canal-boats laded with sufficient stone to sink them. Another steamer, with eight more, will leave in the course of the night. The captain of the Sophia bears a letter to the officer in command of the flotilla, stating "that the boats are to be sunk if necessary." This telegram is sent for your information.

By order of General Meigs, Quartermaster-General:

D. H. RUCKER,

Quartermaster and Colonel.

FORTRESS MONROE, March 9, 1862-6.45 p. m.

Honorable GIDEON WELLES, Secretary of the Navy:

The Monitor arrived at 10 p. m. last night and went immediately to the protection of the Minnesota, lying aground just below Newport News. At 7 a. m. to-day the Merrimac, accompanied by two wooden steamers and several tugs, stood out toward the Minnesota and opened