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._ 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.