This disposition of the sea to cast up well defined boundaries of sand along its margin, is so great and persistent, that the inland waters are dammed up and suffered only to escape into the ocean by narrow avenues, where their rapid currents maintain a supremacy of power--albeit with unceasing contest. Wherever, along our coast, the waves drive _obliquely_ upon the beach, a movement of the sand takes place, and the inlets are consequently continually shifting. The Long Island inlets are moving _westward_, and Sandy Hook advances to the _northward_, because the sea rolls in along the axis of the great bay between Long Island and New Jersey, and necessarily sweeps along the beaches, instead of taking the direction of a _normal_ to the shore line. The movement of Sandy Hook to the northward is, however, a problem not so easily disposed of as we might conceive from the above considerations; for although, in the most general sense, its existence must be regarded as the work of the waves, there are other agents influencing materially its form and its rate of progress. The currents control, to a very great extent, the final disposition of the sands worn away or kept in motion by the waves. Professor Bache's investigations in the neighborhood of Sandy Hook have been published, and we should not especially refer to them here, except that the recent physical changes reported by Colonel Delafield to the Engineer Department, have reawakened an interest in the matter. The measurements of the Coast Survey, made in 1856 and 1857, showed that the Hook was being washed away on the east and west shores, but was extending slowly to the northwest, where it already encroached on the main ship channel. This order of things has continued up to the present time, and is now in progress. The able Superintendent of the fortifications at Sandy Hook has evinced considerable alarm lest the new fort shall fall a prey to the encroachments, or be separated from the main body of the beach by slue-ways. The Coast Surrey has been notified of the matter, and the assistant to whom I have already referred has visited the Hook, and made an informal report, which agrees essentially with the statements of Colonel Delafield. A complete and reliable report can only be made upon _actual surveys_; and we trust these will be executed, and the Government placed in possession of the whole truth. We understand that Colonel Delafield has already, upon a small scale, made some very successful experiments of curvilineal dikes, constructed with caissons of concrete; and we have no doubt that, with adequate means at his disposal, this ingenious engineer could avert the dangers which threaten, not only the fort, but the noble harbor of New York.