HEADQUARTERS NINTH ARMY CORPS, July 26, 1864.

Major-General HUMPHREYS,
Chief of Staff:

GENERAL: I have the honor to acknowledge the receipt of your notes of this morning by Captains Jay and Bache,also of a telegram from the commanding general relating to the same subject. It is altogether probable that the enemy are cognizant of the fact that we are mining, because it has been mentioned in their newspapers and they have been heard to work on what are supposed to shafts in close proximity to our galleries, but the rain of night before last no doubt filled theirs shafts and much retarded their work. We have heard no sound of work in them either yesterday or to-day, and nothing is heard by us in the mine but the usual sounds of work on the surface above. This morning we had some apprehensions that the left lateral gallery was in danger of caving in from the weight of the batteries above it and the shock of their firing, but all possible precautions have been taken to strengthen it and we hope to preserve it intact. The placing of the charges in the mine will not involve the necessity of making a noise. It is therefore probable that we will escape discovery if the mine is to be used within two or three days. It is nevertheless highly important, in my opinion, that the mine should be exploded at the earliest possible moment consistent with the general interests of the campaign. I state to you the facts as nearly as I can, and in the absence of any knowledge as to the meditated movements of the army I must leave you to judge the proper time to make use of the mine. But it may not be improper for me to say that advantages reaped from the work would be but small if it were exploded without any co-operative movement. My plank would be to explode the mine just before daylight in the morning or about 5 o'clock in the afternoon, mass the two brigades of the colored division in rear of my line in columns of divisions, "double column closed in mass," the head of each brigade resting on the front line, and as soon as the explosion has taken place move them forward with instructions for the division to take half distance, and as soon as the leading regiments of the two brigades pass through the gap in the enemy's line, the leading regiment of the right brigade to come into line perpendicular to the enemy's line by the right companies "on the right into line wheel," the left companies "on the right into line," and proceed at once down the line of the enemy's works as rapidly as possible, the leading regiment of the left brigade to execute the reverse movement to the left, moving up the enemy's line. The remainders of the two columns to move directly toward the crest in front as rapidly as possible, diverging in such a way as to enable them to deploy into column of regiments, the right column making as nearly as may be for Cemetery Hill. These columns to be followed by the other divisions of this corps as soon as they can be thrown in. This would involve the necessity of relieving these divisions by other troops before the movement, and of holding columns of other troops in readiness to take our place on the crest in case we gain it and sweep down it. It would be advisable, in my opinion, if we succeed in gaining the crest, would be advisable, in my opinion, if we succeed in gaining the crest, to throw the colored division right into the town. There is a necessity for the co-operation at least in the way of artillery of the troops on my right and left. Of the extent of this you will necessarily be the judge. I think our chances of success in a plan of this kind are more than even.