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.
To return to the Physical Survey, and to speak as briefly as we may upon so extended a subject, we hold that it is possible, by a patient collection of facts and figures, to determine the natural scheme of the harbor—we had almost said the formula of its development.
It is ascertained that the group of shoals which form the Bar—composed, as they are, for the most part, of loose and shifting sands—are not accidental accumulations, modified by violent storms and freshets, but that they are orderly arrangements, made by the currents, to whose unceasing activities are due the form and preservation of each bank and channel. The peculiar contours of the shoals given by our most ancient charts are still developed by recent surveys, although alterations in magnitude have taken place. The order of the physical forces is unchanged, but their work is still progressing.
Now, since these currents have determinable laws, regulating their periods, durations, velocities, and directions, it was only necessary to compile observations, in order to reduce this study to a simple consideration of the composition of forces.
'The process by which sand is swept along by currents upon the bottom of the sea, is not unlike the motion of dunes upon the land; a ridge of sand is propagated in the direction of the current by the continual rolling of the particles from the rear to the front. This movement is exceedingly slow when compared with that of the current which induces it, and for this reason a shoal, though traversed by violent tidal currents, may, as a whole, remain stationary when the alternate drifts are equal and opposite; for in this case, though the sand upon the surface is drifted to and fro, it undergoes no more ultimate change of position than it would if the forces which acted upon it were simultaneous and in equilibrium.'[9]
Of course, so simple a case as that in which the ebb and flood forces are equal and opposite, is rarely presented; for at most of the stations on the Bar the direction of the flow varies from hour to hour, going quite round the circle in a half-tidal day: the velocities and directions also vary with the depth. These circumstances complicate the computation a little, but the problem is still simple and direct. Everything depends upon the faithfulness of the observations.
The physical diagrams which have been plotted from the results of these studies may be regarded as decided successes, for they show in most cases that the shoals lie in the foci or in the equilibrium points of the observed forces.
The current stations occupied cover a district embracing not only the immediate vicinity of the shoals, but extending many miles from them in different directions; for it was deemed necessary that each elementary force should be separately studied before it reached its working point. It has been ascertained that among the causes of the different shoal formations there exists a mutual relation and dependence, so that they may be regarded as a single physical system. It will be seen from this consideration, that any artificial disturbance of the conditions, at a single point, may interrupt the operations of nature in other localities more or less remote, or cause general changes in the hydrogaphy of the harbor.