HEADQUARTERS ARMY OF THE POTOMAC, MORRISVILLE, VA., April 28, 1863. Commanding Officer Cavalry Corps. I am directed by the major-general commanding to inform you that the instructions communicated for your government on the 12th instant, are so far modified as to require you to cross the Rappahannock at such points as you may determine between Kelly's and Rappahannock Fords, and for a portion of your force to move in the direction of Raccoon Ford and Louisa Court House, while the remainder is engaged carrying into execution that part of your original instructions, which relates to the enemy's forces and positions on the line of the Alexandria and Orange Railroad, and the line itself; the operations of this column to be considered as masking the column which is directed to move, by forced marches, to strike and destroy the line of the Aquia and Richmond Railroad. You are further directed to determine on some point for the columns to unite; and it is recommended that it be on the Pamunkey, or near that line, as you will then be in position with your full force to cut off the retreat of the enemy by his shortest line. In all other respects your instructions, as before referred to, will remain the same. You will direct all your force to cross to-night, or, if that shall not be practicable, to be brought to the river, and have it all thrown over before eight o'clock to-morrow morning. If the fords should be too deep for your pack-animals and artillery, they will be crossed over the bridge at Kelly's Ford. You will please furnish the officers in command of these two columns with a copy of this, and of your original instructions. Very respectfully, your obedient servant, WM. L. CANDLER, Captain and Aide-de-Camp. These two orders would appear to be specific enough. The first is not modified by the second to any great extent; and the primary object of both is unmistakably to interrupt, by a bold stroke, Lee's main communications with Richmond by the Fredericksburg and Richmond Railroad. The point on which the two columns, spoken of in the order of April 28, were to unite, was suggested as somewhere on the Pamunkey; and the one column was to go at once about its work, while the other masked its march, and after joined it. Under these orders, Stoneman proceeded to get the corps together,--the distance of many outlying pickets delaying him almost a day,--and finally crossed the Rappahannock by five P.M. of the 29th, a portion of his troops using Kelly's Ford, in connection with Slocum's column. He then assembled his division and brigade commanders, spread his maps before them, and made them acquainted with his orders and plans.