TULLAHOMA, April 6, 1863.

General S. COOPER:

I informed you yesterday by telegraph that General Van Dorn had just reported that a spy brought him intelligence that large re-enforcements are expected for General Rosecrans; that Lieutenant-General Pemberton telegraphed on the 3rd that a scout near Memphis reported, April 1, "All boats pressed; thirty went down empty on Saturday and Sunday;" he adds, "Possibly intend re-enforcing Rosecrans;" and that a gentleman, who left Louisville on the 28th ultimo, says that the Federals were sending all the boats to be had to bring General Grant's troops into Tennessee.*

These three reports corroborate each other, and it seems to me unlikely that General Grant's troops can be intended for any other field than this. Should any large part of his army join that now opposed to General Bragg, the latter must be expelled from Middle Tennessee. Rosecrans' army is now almost double General Bragg's in number, and its position is fortified. It would be madness, therefore, to attack it.

The position of our troops is disadvantageous, because for subsistence it is compelled to take ground west of the direct route from Murfreesborough to Chattanooga. It can, therefore, be turned by our right. At present I do not regard that, because in the event of a large demonstration on that side, we could attack the enemy's main body with strong chance of success, or, if he made the attempt with his whole army, we might exchange bases with him very advantageously. It is now to be feared that the re-enforcements from the Mississippi may enable him to threaten our communications, nd at the same time confront us with a force too strong to be attacked.

Should we be compelled to abandon Middle Tennessee, it will be difficult to feel this army; the cavalry (amounting to nearly 15,000) could not be kept together in East Tennessee or Georgia; it would be necessary to divide it into several bodies. If we had the means of crossing large rivers, I would, in the event of its being compelled to move, send the whole army into West Tennessee to co-operate with that of Mississippi.

Such a movement would be attended with great risk, however, as we could prepare no supplies for the troops before their passage of the river, that district being occupied by the enemy. Should the enemy be too strong to be met in the field, we must choose, it seems to me, between crossing the Tennessee near Chattanooga, to defend East Tennessee and Georgia, and, crossing the river near the Muscle Shoals, to move into Northern Mississippi and West Tennessee, where the army could probably find provisions for several months, and be in position to return if the enemy should send a large force to the east or southeast. The difficulty of the passage of the river is the objection to such a course.

I have (by telegraph) directed Lieutenant-General Pemberton to send Stevenson's division, or an equal number of troops, to this army, if he finds that Grant transfers any large force to Rosecrans. Such movements of the enemy by water can be made in a fourth of the time which we require.

J. E. JOHNSTON,

General.

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*See Seddon to Lee, April 6, 1863, Series I, Vol. XXV, Part II, p. 708.

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