TIDAL WATERS AND CURRENTri. The partial streams of water set In motion by the great tidal waves, which frequently flow in directions different from, or even directly opposed to, the advance of the great tide waves themselves, are known technically by the name of tidal currents ; and they are of the greatest importance in all nautical, or hydraulic, engineering operations on account of the interference they are able to produce in the conditions of access to harbours on the gee-vast, or to the embouchitree of rivers. These currents are pro duce() either by the projection of he all-lands, or by the retarding influence of the sea-shore upon the advance of a portion of the tide wave, and they are therefore subject to so many modifying conditions, as to render it more easy to explain their nature by reference to some of the most remarkable currents known to exist. A few illustrations of them will therefore be given ; but before ao doing, it may be desir able to observe that the great oceanic, currents, such as the Gulf stream, the current through the straits of Gibraltar and at the head of the Mediterranean, &e., have no connection with the class of phenomena in question ; for their creation and propagation depend upon costnieal causes of a very different and of a much more complicated nature than those which produce the real tidal currents of the sea shores.
Now, the great flood tide, following the impulsion it receives from the attraction of the sun and the moan, and from the rotation of the earth, advances normally from the west towards the east ; and in mid ocean the only tidal current is precisely in this uniform direction. On approaching the shores of a large island, or of a continent, however, the tide striking the advanced headlands is diverted from its course in some MUM, whilst In others, the great advancing stream may be carried in its former direction past the portion of the coast immediately behind the headland, and thus only allow the flood tide to exhibit. Itself by derivation from the main stream. On the shores of the Atlantic, for instance, the flood wave striking the extremities of Ireland, Englana, and the department of the Finisterre in France, changes its course in a remarkable manner. A portion of the flood continues to advance in its original direction through the British Channel ; a second portion runs northwards along the western coast of Ireland and of Scotland, giving off in its advance a branch, or subsidiary, current running up through the St. George's Channel, and flowing In a rather north-easterly direction until It tneets a smaller derived current, flowing through the Mull of Cantle), from the N.W. to the fi.E.; a third portion striking
Cape Fiulaterre turns towards the south, and runts along the coast of the Ray of Biscay. The flood title running up the Brit's& Channel passes through the straits of Dover, and spreads itself over the North Sea, meeting a portion of the flood which had paned round the ex tremity of Scotland, and down the eastern coast of England, near the embouchure of the Thames ; so that the united titles flow up that river, sometimes /synchronously, sometimes at slight intervals. In the former case, the tide iu the Thames is a simple ono; In tho latter, it may present the peculiarity of a double rise in the level of the water ; and the same phenomenon of the existence of more than one period of flood way bo perceived In other rivers of the east, and oven of the south, coasts of' England, as well as on the northern coast of France, in consequence of the creation of subsidiary tidal currents by advancing headlands.
Thus, the Bill of Portland projects so much into the lino of advance of the flood tide that the latter sweeps along in the offing at an eleva tion above the water inshore, until it reaches the Needles point, of course parting with seine of its own body, by derivation, into the sheltered bay. At the Needles the flood divides, ono portion flowing from the W. by S. into the Southampton Water, the other continuing up the Channel, but as it passes the Spithead Passage it off a branch Current, which flows through that channel from S.L. to N.W., until it meets the ebbing tide from the Southampton Water already running out. The Spithead flood current drives back the Southampton ebb, and thus makes the peculiar double tide of that river; and it also superposes itself in a manner upon the ebb of the coast lying west of Calehot Point so as to create a double tide, known locally by the name of the Guider, which only ceases to be felt at the Bill of Portland. Very much the same kind of action may be observed on the French coast between the rocks of the Calvados and the Cape Antifcr ; for the first flood tidal current in the bays at the mouths of the Onto and of the Seine, derived from the edges of the great flood tide in the offing, is maintained for a variable period, by the return of a aubaidiary tidal current deflected by the projection of the cape. At Southampton, the double tido keeps the water at nearly a constant height for the epaco of two hours; at Havre it remains in this state for about one hour and a quarter ; whilst at the mouth of the Orne, the flood tide is held up for about one hour. The advantage thus produced to the naviga tion of those localities by these peculiar tidal currents is enormous.