Morganza and 280 feet at New Orleans. This bridge is much worn and requires considerable repairs. There is also a new canvas-boat bridge at New Orleans, 600 feet long, with wagons and tools complete. A few alternations are required, which will be made in a few days. There will be required for this bridge fifty 6-mule teams, six 4-mule teams, harness, and six army wagons. I have now employed quite a large topographical force of engineers and draughtsmen, which has been necessary in this department. They are at present employed in making surveys and plotting notes recently taken in the field. I propose to reduce this force as soon as this work is finished. Almost all the topographical information that it is possible to obtain of this department has been compiled, and is to be found on the maps in this office. Of the country now actually controlled by our forces we have now nearly all the information necessary for military purposes, and surveys are now going on which will render this complete.

The field-works now occupied by our forces in Louisiana are as follows:

Port Hudson.- The enemy's line of works has been repaired and somewhat strengthened, and arranged for field artillery. An interior and very strong line of works has been constructed, designed for a garrison of 2,500 or 3,000 men. This line is finished, as far as its defensive properties are concerned, but considerable work is yet required to properly finish the works.

Baton Rouge.-The grounds of the U. S. arsenal are inclosed by an irregular parapet. This line was first laid out by General Weitzel and has since been strengthened.

Plaquemine.-A work was commenced this spring by the commanding officer of the post and is now nearly finished.

Donaldsonville.-There is a very strong recount at this place.

Brashear City.-There is a square, bastioned fort at this place, large enough to hold a garrison of 800 men, provided with bomb-profess, magazines, and a well.

Berwick City.-There is a large and very strong tete-de-pont constructed on the west bank of Berwick Bay, which was originally built to cover the retreat of our army from New Iberia last fall, when it was threatened by superior numbers. There are also some small defensive works above the Opelousas railroad.

New Orleans.-The land approaches to the city are defended by the lines constructed by the rebels, which have been somewhat strengthened and modified since their occupation by our forces. The lines I consider sufficient to hold the city in the present position of affairs. In case of a large reduction of our force, some additional works and arrangements would be required to insure safety. I will communicate more in detail on this subject in a subsequent report . It may be remarked that the navy can defend New Orleans, having complete control of the river, more effectually than is the case with any other position of equal importance.

In regard to the "posts in this department necessary to be held, and the strength of the garrisons required in each," I would report as follows: The two main routes of approach of an army from Western Louisiana are, first, by Brashear City, and, second, by the upper Atchafalaya, crossing at Simsport or other points above the mouth of the Courtableau, following the Grossetete Valley to Indian Village, crossing Plaquemine and moving by different routes to the La Fourche. The latter route was that taken by the