and Boonsboro until Tuesday night (16th September). McClellan then
ordered Couch's division to be sent to occupy Maryland Heights and
observe the enemy in Harper's Ferry, whilst Franklin with Smith's
and Slocum's divisions should march to the battle-field at daybreak
of Wednesday. Why could not Couch be called up and come on our left
as well as A. P. Hill's division, which was the last of the
Confederate troops to leave the ferry, there being nothing to
observe after it was gone? Couch's division, coming with equal pace
with Hill's on the other side of the river would have answered our
needs as well as one from Porter's corps. Hill came, but Couch did
not. Yet even then, a regiment of horse, watching that flank and
scouring the country as we swung forward, would have developed
Hill's presence and enabled the commanding general either to stop
our movement or to take the available means to support it. The
cavalry was put to no such use. It occupied the centre of the whole
line, only its artillery being engaged during the day. It would have
been invaluable to Hooker in the morning, as it would have been to
us in the afternoon.

McClellan had marched from Frederick City with the information that
Lee's army was divided, Jackson being detached with a large force to
take Harper's Ferry. He had put Lee's strength at 120,000 men.
Assuming that there was still danger that Jackson might come upon
our left with his large force, and that Lee had proven strong enough
without Jackson to repulse three corps on our right and right
centre, McClellan might have regarded his own army as divided also
for the purpose of meeting both opponents, and his cavalry would
have been upon the flank of the part with which he was attacking
Lee; Porter would have been in position to help either part in an
extremity or to cover a retreat; and Burnside would have been the
only subordinate available to check Lee's apparent success. Will any
other hypothesis intelligibly account for McClellan's dispositions
and orders? The error in the above assumption would be that
McClellan estimated Lee's troops at nearly double their actual
numbers, and that what was taken for proof of Lee's superiority in
force on the field was a series of partial reverses which resulted
directly from the piecemeal and disjointed way in which McClellan's
morning attacks had been made.

The same explanation is the most satisfactory one that I can give
for the inaction of Thursday,