It is not simply the superficial drift of the tidal and other currents that these observations comprehend; but, with the use of apparatus suitably arranged, the movements at all depths have been determined, with the exact amount of power exerted by streams coursing along the bed of the sea. The necessity for this minuteness of examination has been fully shown in some of the curious discoveries that have been made.
In several parts of our harbor, systems of counter-currents have been detected, occupying strata of water at different depths, and these present, in their motions, striking contrasts of directions, velocities, and epochs. The most remarkable exhibition of these sub-currents was observed in the neighborhood of the city, in the channel between Governor's and Bedloe's Islands. In this locality, during the last quarter of the ebb, floating objects drift southward toward the sea, while the heavier material upon the bottom is borne northward toward the city piers. While, upon the surface, the ebb exceeds the flood both in velocity and duration, the motions of the lowest water-stratum are subject to the reverse conditions: it therefore follows that the heavier deposits from the city drainage cannot be swept away through this the main avenue to the sea. This contrast of motion between the upper and lower drifts was observed in greater or less degree throughout the entire distance from the Bar to a point in the Hudson River off Fort Washington. These results appear to us of the highest importance, since they would seem to indicate that the scouring action of the currents will not be sufficient to prevent the accumulation of certain classes of deposits in the upper harbor—as the ashes from the steamers, and the like.
The course of the land waters in their progress seaward was followed nearly sixty miles beyond the Bar, where currents of considerable velocity were still observed. At the station farthest seaward, where the sounding is thirty fathoms, the observations at different depths disclosed some very remarkable peculiarities. It was perceived that the moving stratum was not always of the same depth; the whole body of the sea moving steadily forward at one time, while at another no motion could be detected below a superficial stream.
The land waters, to which allusion has been made, augment the ebb current to such a degree, that a general eastwardly preponderance was observed in the drift along the south shore of Long Island; and this preponderance, increasing steadily from station to station at each remove, was found, at a point twenty-five miles east of Fire Island Light, to outlive the tidal currents and maintain itself as a constant coastwise stream.
One very curious discovery was made with regard to this stream along shore. It was ascertained that during easterly gales a portion of the water, crowded up into the bight of the coast, escapes seaward by a sub-current. Shells, carefully marked, were deposited in the sea during fine weather, and, after an easterly gale, were picked up on the shore of Fire Island, four miles eastward of the place of deposit. There was no evidence that these shells travelled any distance during still weather.
We do not despair of the possibility of artificial improvements at the Long Island inlets.
At present the great inland basins on the southern portion of Long Island communicate with the sea only by narrow passes obstructed by bars and shoals; yet, in spite of the dangers which are always presented, large fleets of market vessels pass out daily through the inlets, laden with farm produce and shell fish. It requires no thought to perceive that if these inlets were made safe and permanent by suitable marine constructions, and were furnished with the proper buoys and beacons, there would spring up in their neighborhood great commercial enterprises.
While, in the case of the lower harbor and its approaches, it was the design of the observations to detect in the movement of the waters the causes of alterations in the physical geography, the same kind of studies, undertaken afterward in Hell Gate, had for their object the reverse inquiry, viz., to ascertain to what degree and in what manner the form of the rocky channel influenced the tides and currents, in order that some prediction might be made of the consequences likely to follow the removal of obstructions from the waterways. The propagations of the tide wave meet at Hell Gate, so that here the observations, when plotted, exhibit compound curves, in which the portion due to the wave from Sandy Hook is easily distinguishable from that due to the wave from Long Island Sound. The Sandy Hook tide wave differs so widely in height and time from that of Long Island Sound, that there is over three feet difference of level between the harbor and the Sound at certain stages of the tides; and at these times the currents rush through the Gate, vainly endeavoring to restore the inequalities.
The problem of referring a current to a tidal head is a very difficult one. The current, for instance, which renders Hell Gate so dangerous, is not at any time so great as a permanent head, equal to the difference of the tides observed, would engender. The currents are so very slow in their movements, compared with the undulations of the tide wave, that it cannot be ascertained as yet, what are the magnitudes of such elements as inertia and friction, and how they are to be corrected for, so as to predict the time and velocity of the current from observations of the vertical rise and fall.
It is due to the officers of the Coast Survey to state that their services to the Harbor Commissioners were rendered gratuitously; the work offered to them only an opportunity for research.