In connection with the surveys made a few years since by direction of the Commissioners on Harbor Encroachments, there was undertaken, as an incidental inquiry, an investigation into the physical conditions under which the shoals and beaches in and about New York harbor had submitted to those changes of position and area which the repeated surveys revealed. It was at the request of these Commissioners that Professor Bache, the Superintendent of the Coast Survey, gave his personal attention to this subject. He drew up a comprehensive scheme for a series of observations upon all the natural agencies at work, and, for the execution of the project, selected one of his assistants, whose experience had already been considerable in similar studies. The investigation was commenced in the lower harbor early in the spring of 1856. Records were kept of tides, currents, winds, and waves, and the most careful notes were made on the immediate effects of these working agents as observable in the movements of the sands. A glance at a general coast chart discovers at once a marked contrast between two different sections of our seaboard: to the eastward of us, the principal harbors of New England are rockbound, with elevated back countries; while to the southward, in the region of alluvial drift, which extends all along the coast of the Middle and Southern States, the harbors have flat and sandy shores. The harbor and neighborhood of New York, holding an intermediate position between these diverse sections, exhibit a singular combination of the leading physical features of both, and present to the hydrographer a field for research that is quite without a parallel. We recognize in the Bar of New York simply a submerged portion of that _sandy cordon_ which skirts the coast from Montauk Point to Florida; and although, in the ordinary sense, the lower entrance to the harbor is not an _inlet_, it may nevertheless be regarded as belonging to the same class. This _sandy cordon_, which may be said to be the principal characteristic of our coast, is an exceedingly interesting feature; it appears to have been formed by the action of the sea, which has disintegrated the borders of shallow flats, bearing away the light vegetable moulds, but suffering the coarse quartz sand to remain rolled up into ridges. In many places the dry winds have caught up these sands, when laid bare at low water, and elevated them into dunes or galls. The distance of the sand ridge from the mainland is observed to vary with the slope of the adjacent country. It is the _motion of translation_ which a wave acquires on reaching shallow water, that gives it such great capacity for the transportation of material. This _translative action_, as it is technically called, commences ordinarily in about three fathoms water, and is most violent in six or eight feet depths, within which the sea breaks. It is just within the breaker that the windrows of sand are observed to form on exposed flats.