SIR—On the morning of the 21st I received a communication from General Grant, informing me that he intended to attack the whole of the rebel works at ten A. M. the next day, and asking me to shell the batteries from half-past nine until half-past ten, and to annoy the garrison. I kept six mortars playing rapidly on the works and town all night, and sent the Benton, Mound City and Carondolet up to shell the water batteries and other places where troops might be resting during the night. At seven o’clock in the morning, the Mound City proceeded across the river, and made an attack on the hill batteries opposite the canal. At eight o’clock I found her in company with the Benton, Tuscumbia, and Carondolet. All these vessels opened on the hill batteries, and finally silenced them, though the main work on the battery containing the heavy rifled gun was done by the Mound City, Lieutenant Commanding Byron Wilson. I then pushed the Benton, Mound City and Carondolet up to the water batteries, leaving the Tuscumbia, which is still out of repair, to keep the hill batteries from firing on our vessels after they had passed by. The three gunboats passed up slowly, owing to the strong current, the Mound City leading, the Benton following, and the Carondolet astern. The water batteries opened furiously, supported by a hill battery on the starboard beam of the vessels. The vessels advanced to within four hundred and forty yards (by our marks) and returned the fire for two hours without cessation, the enemy’s fire being very accurate and incessant. Finding that the hill batteries behind us were silenced, I ordered up the Tuscumbia to within eight hundred yards of the batteries; but the turret was soon made untenable, not standing the enemy’s shot, and I made her drop down. I had been engaged with the forts an hour longer than General Grant asked. The vessels had all received severe shots under water which we could not stop while in motion, and not knowing what might have delayed the movement of the army, I ordered the vessels to drop out of fire, which they did in a cool, handsome manner. This was the hottest fire the gunboats had ever been under; but, owing to the water batteries being more on a level with them than usual, the gunboats threw in their shell so fast that the aim of the enemy was not very good. The enemy hit our vessels a number of times, but fighting bow on, they did but little damage. Not a man was killed, and only a few wounded. I had only enough ammunition for a few moments longer, and set all hands to work to fill up from our depot below.