Of the progress which has been made in the construction of fortifications for the permanent defense of our maritime frontier, according to the plan decided on and to the extent of the existing appropriations, the report of the Secretary of War, which is herewith communicated, will give a detailed account. Their final completion can not fail to give great additional security to that frontier, and to diminish proportionably the expense of defending it in the event of war. The provisions in several acts of Congress of the last session for the improvement of the navigation of the Mississippi and the Ohio, of the harbor of Presqu'isle, on Lake Erie, and the repair of the Plymouth beach are in a course of regular execution; and there is reason to believe that the appropriation in each instance will be adequate to the object. To carry these improvements fully into effect, the superintendence of them has been assigned to officers of the Corps of Engineers. Under the act of 30th April last, authorizing the President to cause a survey to be made, with the necessary plans and estimates, of such roads and canals as he might deem of national importance in a commercial or military point of view, or for the transportation of the mail, a board has been instituted, consisting of two distinguished officers of the Corps of Engineers and a distinguished civil engineer, with assistants, who have been actively employed in carrying into effect the object of the act. They have carefully examined the route between the Potomac and the Ohio rivers; between the latter and Lake Erie; between the Alleghany and the Susquehannah; and the routes between the Delaware and the Raritan, Barnstable and Buzzards Bay, and between Boston Harbor and Narraganset Bay. Such portion of the Corps of Topographical Engineers as could be spared from the survey of the coast has been employed in surveying the very important route between the Potomac and the Ohio. Considerable progress has been made in it, but the survey can not be completed until the next season. It is gratifying to add, from the view already taken, that there is good cause to believe that this great national object may be fully accomplished. It is contemplated to commence early in the next season the execution of the other branch of the act--that which relates to roads--and with the survey of a route from this city, through the Southern States, to New Orleans, the importance of which can not be too highly estimated. All the officers of both the corps of engineers who could be spared from other services have been employed in exploring and surveying the routes for canals. To digest a plan for both objects for the great purposes specified will require a thorough knowledge of every part of our Union and of the relation of each part to the others and of all to the seat of the General Government. For such a digest it will be necessary that the information be full, minute, and precise.