HEADQUARTERS CHIEF ENGINEER OF DEFENSES,
Washington, July 6, 1864.

Major General H. W. HALLECK,

Chief of Staff:

SIR: in compliance with your instructions of yesterday I proceeded last evening to make an examination of Chain Bridge and Aqueduct Bridge, and this morning I sent Mr. Childs, our civil assistant in charge of the works south of the Potomac, to make an examination of the Long Bridge, and i now have the honor to submit the following report on the defenses and guards of the tree bridges across the Potomac, viz:

Chain Bridge. - The defenses of this bridge on the Virginia side consists, as you are aware, of a large tete-de-pont, comprising Forts Marcy and Ethan Allen, with a connecting line of intrenchments which extend also from the right of Fort Marcy and left of Fort Ethan Allen to the bank of the river above and below the bridge. Across the Leesburg pike, immediately on the right of Fort Marcy, there is a strong stockade with gates. This is a partial security against a sudden dash of cavalry in force, but is not a complete one, as there are paths, and, in fact, another road, passing to the left of Fort Ethan Allen leading to the bridge. The immediate defense of the bridge consists in a loop-holed gate which stands on the last pier net the Virginia shore, the whole river, except during floods, flowing between this pier and the adjacent abutment, the planks over a portion of this span being laid down loosely so that they can be taken up at night. At this end of the bridge there is a battery armed with two mountain howitzers, intended to enfilade it, and on the hill immediately above and in rear is Battery Martin Scott, armed with two rifled 6-pounder guns. I found Lieutenant Grunwell, of the Twenty-second Regiment Veteran Reserves, with sixty-three men, in charge of the bridge. He has no command of the batteries at the east end of the bridge. These are in charge of Private Spink, One hundred and forty-seventh Ohio National Guard, acting ordnance sergeant. He knows nothing about ordnance or artillery. In fact, no one at the bridge knows how to load the guns. The business of Sergeant Spink is to clean the guns, air the ammunition, and sweep the platforms. In these duties he is assisted by a detail of six men from the One hundred and forty-seventh Ohio National Guard, changed daily. Lieutenant Grunwell closes the gates at the west end of the bridge and takes up eight or ten flooring planks in front of the gates every night at 9 o'clock.

If the enemy were to attack him suddenly he "would take up the planks and make a parapet or barricade of them, and close the gates. "

He understands that there is a magazine in one of the western piers, arranged for blowing up the bridge, and a small lead pipe leading to the magazine, and steps from the floor of the bridge leading down to where the pipe projects from the pier. In all of which he must be mistaken as I have never that the piers of the bridge are mined.

I remark:

First. That the defenses of this bridge are sufficient, but the guard is not. The charge of the bridge with the two batteries at this end should be under one head. One man should be responsible for everything. He ought to have an ordnance sergeant and sufficient artillerist to ma the four guns.