nce with the
plan, which he has imparted to him in conversation, and in obedience to
the written instructions to: "Observe well the roads to Bull Run and to
Warrenton. Do not bring on an engagement, but keep up the impression
that we are moving on Manassas,"--this advance, by way of Centreville,
being intended solely as a "demonstration" to mask the real movement,
which, as we have seen, is to be made by the other divisions across Wolf
Run Shoals, a point on Bull Run, some five or six miles below Union
Mills, and some seven miles below Blackburn's Ford.

Upon the arrival of Richardson's Brigade, Thursday morning, at
Centreville, it is found that, under cover of the darkness of the
previous night, the Enemy has retreated, in two bodies, upon Bull Run,
the one along the Warrenton Pike, the other (the largest) down the
ridge-road from Centreville to Blackburn's Ford. Richardson's Brigade
at once turns down the latter road and halts about a mile beyond
Centreville, at a point convenient to some springs of water. Tyler soon
afterward rides up, and, taking from that brigade two companies of light
Infantry and a squadron of Cavalry, proceeds, with Colonel Richardson,
to reconnoitre the Enemy, finding him in a strong position on the
opposite bank of Bull Run, at Blackburn's Ford.

While this is going on, McDowell has ridden in a Southerly direction
down to Heintzelman's Division, at Sangster's Station, "to make
arrangements to turn the Enemy's right, and intercept his communications
with the South," but has found, owing to the narrowness and crookedness
of the roads, and the great distance that must be traversed in making
the necessary detour, that his contemplated movement is too risky to be
ventured. Hence he at once abandons his original plan of turning the
Enemy's right, and determines on "going around his left, where the
country is more open, and the roads broad and good."

McDowell now orders a concentration, for that night, of the four
divisions, with two days cooked rations in their haversacks, upon and
about Centreville,--the movement to commence as soon as they shall
receive expected commissariat supplies. But, later on the 18th,
--learning that his advance, under Tyler, has, against orders, become
engaged with the Enemy--he directs the concentration to be made at once.

Let us examine, for a moment, how this premature engagement comes about.
We left Tyler, accompanied by Richardson, with a squadron of Cavalry