ORDINARY TIDES General Description of Phenomena.—(2). At most seaside places the water reaches its highest level approximately twice a day, the average interval between two successive high waters being 2 hr. 25 min., though this interval varies considerably during the course of a week. At certain places in the East Indian Seas two successive high waters are separated by an interval of 12 hr., while at certain places in the China Sea the interval is often over 24 hr. At places on the shores of the oceans the time taken by the tide in rising is about equal to the time taken in falling on the same day, but in estuaries the tide usually rises quicker than it falls. At certain places, such as Southampton, England, the high waters are often doubled, i.e., the water reaches a maximum height, falls a little and then rises to a maximum again. At other places the low waters are often doubled. As we go along any stretch of coast the time of high water generally becomes progres sively earlier or later, while as we go up an estuary from the sea the time of high water always becomes progressively later.
At most places, on the average, a high water is about as much above the mean level of the sea as the succeeding low water is below it. The difference in level between successive high and low waters is called the range of tide. The range of tide at any place may vary much from day to day. At most places it reaches a maximum once a fortnight, and a minimum at times midway between two successive maxima. At London Bridge the greatest range of the fortnight has an average value of 21 ft. o in. while the least range of the fortnight has an average value of 14 ft. II in. At the head of the Bay of Fundy (q.v.) the range of tide reaches 5o ft. while at certain islands of the Pacific and over most of the Mediterranean the range never exceeds 2 ft. At many places outside the Atlantic the heights of two successive high or low waters are markedly different, a phenomenon known as diurnal inequality.
At a place in a strait or narrow sea the tidal current usually flows for about 6 hr. 12 min. in one direction, and then for about the same time in the opposite direction. At the reversal of such a current we have the state of rest usually called slack water. In estuaries the current generally flows down-stream for a greater length of time than it flows up-stream. At a place in the open sea the direction of the current often takes all points of the com pass, making the complete revolution in the tidal period. During this period there are usually two times of maximum current and two intermediate times of minimum current. The times of maximum current are separated by about half the tidal period and the directions of maximum current are nearly directly op posite; similarly for the two minimum currents. There is no fixed
general relation between the time of high water and the time of maximum current. If the current flows directly in and out of a bay it will reach its inward maximum nearly a quarter period before high water at the head of the bay, so that slack water is simultaneous with high water. On the other hand, in an estuary the current continues to flow up-stream for some considerable time after high water and to run down-stream similarly after low water. When the current is directed towards land or up an es tuary it is called the flood current ; when it runs away from land or down an estuary it is called the ebb current. The speeds of tidal currents vary greatly from place to place; e.g. in the Mersey at Liverpool the maximum current reaches 7 knots, while in the North Sea it rarely exceeds i knot. At some distance up certain rivers—as, for example, the Severn—the rising water spreads over the flat sands in a roaring surf and travels up the river almost like a wall of water: This phenomenon is called a bore. Near a headland separating two bays there is sometimes a very swift current termed a race.