WASHINGTON, D.C., July 16, 1861.
Honorable GIDEON WELLES,
Secretary of the Navy:

SIR: We have the honor to inform you that one of the results of our study of the Southern Atlantic coast of the United States in reference to its blockade is to recommend that it be divided into two sections, one of which will extend from Cape Henry to Cape Romain, about 370 miles, and the other from Cape Romain to St. Augustine, about 220 miles. The geographical features of the northern or upper section are very different from those of the southern or lower section, and accordingly the treatment of the two should be distinct. The former is characterized by narrow belts of sand which separate large inland waters from the ocean, and are divided at irregular intervals by openings or inlets through which the ocean tides ebb and flow and access is obtained to the enclosed sounds. The latter is distinguished by the ordinary ports and bays. The subject of the present communication will be the first of these sections; the second is reserved for a future report. In order to show the importance of this section of the coast, which embraces the whole seaboard of North Carolina and a portion of that of South Carolina and Virginia, we must observe, in the first place, that the Elizabeth and James rivers (the ports of Norfolk and Richmond) are not effectually blockaded until the entrance into Albemarle, Pamlico, and Core sounds is stopped against the enemy. The external boundary of these sounds, the narrow belt of sand separating them from the sea, is sparsely inhabited, except at summer watering places like Nag's Head, or the few and distant towns like Portsmouth. The inner shore line consists chiefly of marshes and cedar swamps, but the interior communication with Norfolk by the Dismal Swamp Canal and by the Albemarle and Chesapeake Canal and by the rivers and railroads with North Carolina and Virginia is complete. All this is so apparent upon the maps that it is only necessary to allude to it. The several towns of Elizabeth City, Edenton, Washington, and New Berne; the rivers North, Pasquotank, Pamlico, and Neuse, are connected in trade and intercourse by numerous steamers suited to the navigation of these shallow waters. This trade includes a considerable foreign commerce with the West Indies, of no great importance (for revenue) to the United States, but vastly important to the rebels while they are debarred the use of the entrance to the Chesapeake. But for this the sterile or half drowned shores of North Carolina might be neglected.

But it is an important object in the present war that this trade, home and foreign, should be interrupted, and for this purpose it is desirable to adopt some general method by which the approaches from the sea and the channels inside from sound to sound may be shut up.

The most obvious method of accomplishing this object is by putting down material obstructions; and the most convenient form of obstruction, for transportation and use, is that of old vessels laden with ballast in a neighboring port, and sunk in the appropriate places. They would entirely obliterate the old channels; new channels would be formed in time, but their general use would be almost impossible, certainly very precarious, until they were reexamined at leisure. The part of the coast of which we are speaking is peculiarly adapted to this course of action by the smallness of the tides, which have a rise of only 1 feet at Cape Hatteras, 2 feet at Cape Henry, 109 nautical miles north, and 2 feet at Cape Lookout, 68 miles south. They flow but a short distance into Pamlico Sound, where the inner bars or "bulkheads," as they are defined on the Coast Survey charts, mark the places of principal deposit of the moving sands and of meeting, therefore, of the inner and outer waters.

The chief rise and fall of the waters of these sounds are caused by the winds; wherever the obstacles are so placed as to be sheltered from them they would probably remain; but in very severe storms, in which the waters are heaped up on one side of the sound, their return to a state of equilibrium is attended with so much violence that the smaller hulks could hardly fail to be acted upon and removed.

These are extreme cases. There would be no force arising from regular currents, or the momentum of large bodies of water that would probably undermine them, except in heavy gales. Piles might be driven in the interior of the sounds, but their employment is less simple and more laborious. Our sunken hulks should be visited frequently to guard against their removal, but with the means at hand the enemy could effect but little in that way. Besides the inlets, there are two harbors that may have to be blockaded from outside, Beaufort and Cape Fear River, unless expeditions were undertaken to capture Forts Macon and Caswell, which would form a separate subject of consideration. It is said that the alarm of the people of Wilmington has led them to close New Inlet. If, instead of being closed, it should be fortified, the batteries, we imagine, could be easily turned, whether on the main or on Smith's Island. If we carry our operations as far as Georgetown, that harbor may be closed or blockaded, as found most convenient.

This long stretch of coast of about 350 miles, one-third of the whole coast from Cape Henry to Cape Florida, requires the watching or closing of only ten or twelve harbors and inlets. In this respect, and in respect to population, supplies, fertility, or natural resources, it is the weakest part of the coast of the United States of the same extent.

The seaboard itself would furnish but little resistance, except where the inlets have been fortified, and expeditions from the sounds and from the rivers emptying into them must be conducted in small vessels and steamers of light draft, such as would be easily met and discomfited. It is certain that the distance of the seacoast from the interior, and the thinly distributed population, will prevent any rally in time to interfere with our operations.

Wood and water are found on the sand beaches, and the latter may be obtained in some places by boring the cedar swamps. The coast of South Carolina, between the boundary of North Carolina and Georgetown, includes the famous Horry district, the forests and swamps of which come down nearly to the ocean shore. Major Prince, U. S. Army, explored it for the Coast Survey, and the triangulation was carried on by cutting through nearly every step of the way after leaving the sounds south of Cape Fear entrance. The vicinity of Cape Hatteras is one of the worst regions on our coast for tempestuous weather, the cape itself being the point of separation between the storms peculiar to the two divisions of the country--the West India hurricanes at the south, the course of which, after skirting the ocean borders of Florida, Georgia, and South Carolina, turns to the eastward before arriving at Cape Hatteras, and the common northeast storms of our Northern States, which begin at the southwest extremity of their track and make their progress to the northward and eastward. We will now proceed to name these inlets, from north to south, in order in which they stand on our notes, and specify the manner in which they ought, in our opinion, to be treated.

1. Oregon, or New Inlet, 35 miles north of Cape Hatteras, Pamlico Sound, coast of North Carolina, has a dangerous shifting bar, on which there is 7 feet at low and 9 feet at high water. It should be blocked up by sinking as many hulks as necessary for the purpose. The place for these hulks will be shown on the chart.

2. Loggerhead Inlet. Not well known by us.

3. Hatteras Inlet, 11 miles southwest of Hatteras, opening also into Pamlico Sound, has 19 and 21 feet on the ocean bar, with a good, easy entrance; 13 and 15 feet in the Oliver's Channel anchorage, and 7 and 9 feet on the inside bar or bulkhead, which it is necessary to cross to enter the sound. We recommend the final obstruction of the inner bar, reserving the anchorage for a harbor of refuge.

4. Ocracoke Inlet, 14 miles southwest of Hatteras Inlet, with 10 and 12 feet on the outer bar, and a good and easy entrance for 10 feet; has safe anchorage in 19 feet. This inlet may be treated like that of Hatteras.

Before closing the two last inlets on the inside it will be well to decide upon the course to be pursued in relation to the interior navigation. It may be threatened or controlled by a force of small steamers armed with rifled cannon. It may be kept open for the purpose of capturing the towns on the Pasquotank, the Chowan, the Roanoke, the Pamlico, and Neuse rivers; or it may be obstructed by sinking hulks between Core and Albemarle sounds, between Albemarle and Roanoke sounds at Roanoke Island, and elsewhere, as future enquiry may point out. We strongly advise the latter. This whole region of marshes and cedar swamps is fatally unhealthy at this season of the year (except on the immediate seashore) to our Northern constitutions.

5. Beaufort, or old Topsail Inlet, may be both blockaded and obstructed. The main channel has 15« and 18 feet low and high water.

We have already suggested the propriety of taking Fort Macon. The people at Beaufort are said to be loyal. How this may be we have no certain means of knowing.

6. Bogue Inlet, 10 leagues west of Cape Lookout, with 8 feet on the bar; 7, New River Inlet, 14« leagues, with the same depth, and 8, New Topsail Inlet, 20« leagues west of Cape Lookout, with 10 feet over the bar, must all be obstructed.

9. New Inlet, near Cape Fear, as we before mentioned, is said to have been closed by the people of Wilmington; if not, its obstruction can be easily effected.

10. The Western Bar, at the entrance of Cape Fear River, must probably be blockaded. It lies under the guns of Fort Caswell, and unless the fort is taken the sinking of obstructions on the bar will be difficult. This bar has 8 and 12 feet.

11. Lockwood's Folly Inlet, and 12, Tubb's Inlet, may be obstructed.

13. And finally, Georgetown entrance, or Winyah Bay, may be both obstructed and blockaded.

The archives of the Coast Survey will furnish the best information concerning this region of coast.

These plans may undergo some modification in the hands of the person to whom their execution shall be intrusted.

We have the honor to be, very respectfully, your most obedient servants,

S. F. DU PONT,
Captain, U. S. Navy, and President.
A. D. BACHE,
Superintendent U. S. Coast Survey, Member.
J. G. BARNARD,
Major, U. S. Engineers, Member.
CHARLES HENRY DAVIS,
Commander, U. S. Navy, Member and Secretary.