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.