The waters of the ocean are observed to flow and rise twice a day, in which motion, or flux, which in the same direction lasts nearly six hours, the sea gradually swells, and, entering the mouths of rivers, drives back the river-waters toward their head. After a continued flux of six hours, it seems to repose for a quarter of an hour, and then begins to ebb, or retire back, for six hours more; in which time, by the subsidence of the waters, the rivers resume their usual course. After a quarter of an hour, the sea again flows and rises as before. According to the theory of Newton, these phenomena were supposed to be produced by an imaginary power called attraction. The moon was supposed to attract the waters by the influence of an occult power inherent in all matter; just as the earth was supposed to attract the moon, the moon the earth, and the planets one another. Others, again, ridicule this idea, as unsustained and visionary, giving in their turn some theory that has no better argument to sustain it. And it is probable the time is yet future when we shall have any theory that will fully account for all the phenomena of the tides. On account of the shallowness of some seas, and the narrowness of the straits in others, there arises a great diversity in the phenomena, only to be accounted for by an exact knowledge of the place. For instance, in the English channel and the German ocean, the tide is found to flow strongest in those places that are narrowest, the same quantity of water being, in this case, driven through a smaller passage. It is often seen, therefore, rushing through a strait with great force, and considerably raised, by its rapidity, above that part of the ocean through which it runs. The shallowness and narrowness of many parts of the sea, give rise also to a peculiarity in the tides of some parts of the world: for in many places, in the British seas in particular, the greatest swell of the tide is not while the moon is in its meridian hight, and directly over the place, but some time after it has declined thence. The sea, in this case, being obstructed, pursues the moon with what dispatch it can, but does not arrive with all its waters, until after the moon has ceased to operate. Lastly, from this shallowness of the sea, and from its being obstructed by shoals and straits, it happens that the Mediterranean, the Baltic and the Black sea, have not any sensible tides, to raise or depress them in a considerable degree. Among the phenomena of the tides, one of the most singular is the bore, peculiar to several rivers: it is ascribed to the waters, which were before expansive, being suddenly pent up, and confined within a narrow space. This bore or impetuous rush of waters, accompanies the first flowing of the tide in the Ferret, in Somersetshire, and in the Seine, in France. It is also one of the peculiarities of the Severn, the most rapid river in England.