From Harper's Ferry, if it is found that the enemy has moved north of the Potomac in large force, push north, following him and attacking him wherever found; follow him, if driven south of the Potomac, as long as it is safe to do so.

If it is ascertained that the enemy has but a small force north of the Potomac, then push south with the main force, detaching under a competent commander, a sufficient force to look after the raiders, and drive them to their homes.

In detaching such a force, the brigade of the cavalry now en route from Washington via Rockville may be taken into account.
"There are now on their way to join you three other brigades of the best cavalry, numbering at least five thousand men and horses.

These will be instructed, in the absence of further orders, to join you by the south side of the Potomac.

One brigade will probably start to-morrow.