HEADQUARTERS THIRTEENTH ARMY CORPS, Smith's Plantation, April 12, 1863.

Major General U. S. GRANT, Comdg. Department of the Tennessee:

GENERAL: I am at this place, 2 miles from Carthage.

It is reported that the enemy have a force of some 1,200 or 1,500 men, including a battery of four 6-pounder and two 12-pounder guns, at the lower end of Saint Joseph Lake.

By reference to the map of Mississippi, you will see that there is a road leading from Grand Gulf to Warrenton, which crosses the Big Black a short distance above its mouth, and another leading back by Willow Springs to Warrenton, which crosses Big Black higher up. Whether my forces should ascend the Big Black and disembark at the crossing of the first or SECOND of the roads mentioned, or on the Mississippi shore near Grand Gulf, is a question upon which I am not advised. As I have none but imperfect maps, please furnish me with a complete one, with all other information you can afford; also a guide, if you can. A messenger will be found at my headquarters at the Bend to bring anything you may wish to send.

I will order the siege pieces forward with your permission. General Benton, to whose brigade they are attached, says he can bring them. Small craft are to-day bringing commissary stores from Richmond to this place. As soon as they return, they will haul ammunition. On my way here I found a number of excellent flats partially constructed. They will be launched as soon as finished. I have ordered the NINTH and Fourteenth DIVISIONS of my corps to hold themselves in readiness for concentration here, preparatory to embarkation. As my means of supplying my forces with ammunition and other requisite stores are limited, and not altogether reliable, I must trust to you to assist me.

Your obedient servant,
JOHN A. McClernand.

P. S. -I may find it useful to return to Richmond and the Bend to-morrow.

MILLIKEN'S BEND, La., April 12, 1863.

Major General John A. McClernand, Comdg. Thirteenth Army Corps:

I was anxious to have seen you and had a conversation upon present movements before your leaving, if I could have done so. I will, however, probably go over to New Carthage before you get away.

On Tuesday or Wednesday night, Admiral Porter will run the Vicksburg batteries, and I will send, at the same time, four steamers and all the barges then ready, probably twelve. If these all get through safely, you will have 300,000 rations aboard, and transportation, by close packing, for two batteries and from 6,000 to 8,000 men. It is desirable that you should take all the men possible, with the transportation at your hands, on the start.

It is my desire that you should get possession of Grand Gulf at the earliest practicable moment. Concentrate your entire corps there with all rapidity, and, as soon as transportation can be got through for them, move down the river to Bayou Sara. From there you can operate on the rear of Port Hudson, in conjunction with Banks from Baton Rouge. I will write to General Banks, to be sent down by the gunboats, informing him of present plans, and timing our movements as near as possible.