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On Thursday, July 18, 1861, around noon, Union troops under Irvin McDowell approached Centreville, Virginia. Expecting to find the enemy there, McDowell sent a brigade from Daniel Tyler's division, some 3000 men, to reconnoiter. They advanced into Centreville, found it empty, and marched on to Bull Run.
At Blackburn's Ford, one of the many crossings of Bull Run, Tyler ordered Colonel Israel Richardson to probe the position. This was the center of Beauregard's line, and Richardson ran into James Longstreet's brigade hidden in the dense woods south of the stream. Tyler had a hard time disengaging from the enemy, but eventually retreated back to Centreville with news for McDowell about the Confederate postion.
The Battle of Blackburn's Ford, a reconnaissance-in-force, was a fairly meaningless skirmish, a prelude to the much bigger First Battle of Bull Run on July 21. The skirmish led McDowell to decide against a frontal assault on the Confederate line. He would try to flank the left side of the Confederate line, crossing farther upstream to the north.
Also on this date, at Richmond, Virginia, Confederate Secretary of the Navy Stephen Mallory provided a report on the cost of converting the captured U.S.S. Merrimack into an ironclad.
The frigate Merrimack has been raised and docked at an expense of $6000, and the necessary repairs to hull and machinery to place her in her former condition is estimated by experts at $450,000. The vessel would then be in the river, and by the blockade of the enemy's fleets and batteries rendered comparatively useless. It has therefore been determined to shield her completely with 3-inch iron placed at such angles as to render her ball-proof, to complete her at the earliest moment, to arm her with the heaviest ordnance, and to send her at once against the enemy's fleet. It is believed that thus prepared she will be able to contend successfully against the heaviest of the enemy's ships and to drive them from Hampton Roads and the ports of Virginia. The cost of this work is estimated by the constructor and engineer in charge at $172,523, and as time is of the first consequence in this enterprise I have not hesitated to commence the work and to ask Congress for the necessary appropriation.