Mobile. Originally there were three great lines of communication through these States, but one has been lost by the permanent occupation of Chattanooga, and the value of the two remaining is, necessarily, greatly enhanced. Our permanent seizure of them would severely paralyze the Confederacy.

Now, attacks upon the flanks or extremities of this line, from Richmond to Mobile, even if perfectly successful, must be indecisive, if not futile, since the defeated armies are driven upon the center of theirs line, where they would concentrate their strength and avail themselves of interior lines.

Offensive movements, therefore, that can give us this advantage are greatly to be preferred, particularly if they place our armies upon the enemy's communications.

Such movements cannot advantageously be made from Chattanooga. An advance from that point involves a march of 120 miles to atlanta, and thence 80 more to Macon, in the face of a powerful army, holding strong defensive positions, by roads almost impracticable during winter, and with lines of supply so extended that even now they cannot be considered as perfectly secure. But these two routes approach very closely to the Atlantic coast between Charleston and Savannah.

The occupation of Charleston is, however, unessential, since it still leaves open the route by Kingsville, Branchville, and Augusta, and the swampy defiles in rear of Charleston would not easily be forced.

Between Charleston and Savannah are several excellent harbors from which deep streams penetrate far inland, and numerous "sea islands" that would serve as depots and bases. The Eidsto, Ashepoo, Combahee, and Broad Rivers are navigable up to the Charleston and Savannah Railroad. A force suddenly thrown upon this road - which is rather watched than guarded - would appearto be with a view to an attack upon one or both the cities named. An advance of two days would suffice to reach Branchville. This point fortified, with communications protected by the Edisto and Combahee (or Savannah) Rivers, with the Ashepoo between navigable nearly to Walterborough, based upon the railroad from Jacksonborough to Coosawhatchie and the islands already in our possession, and all free communication ceases between the extreme States of the Confederacy.

Forty or fifty thousand men should commence such a movement, and as many more rapidly support it. The northern armies could supply that force as soon as winter forbids active operations. volunteer and drafted troops would defend the Potomac meantime. If necessary, such detachments could return in the spring, although inall probability the battle would be fought entirely in South Carolina, and Lee's army, if not a portion of Bragg's, would be immediately withdrawn to oppose such permanent occupation of the vital lines of communication.

Large number of black would be enabled to enter our lines, and the country is at all seasons of the year exceedingly healthy, being "pine land," and the roads excellent winter and summer, so that operations need never be hindered by the seasons.

These suggestions, affecting a projected theater of operations within this department, are therefore respectfully submitted.

T. SEYMOUR,

Brigadier-General.