Earth's rotation. And the complexity is added to by the irregular 

 shape of the coast lines and the varying depth of the sea floor. The 

 oceans may be regarded as forming natural basins, in each of which 

 the water — once disturbed — would continue to move back and 

 forth, each basin having its own period of oscillation. If the period 

 of oscillation happens to be close to the tidal period, then the 

 response of the water is much increased. The large semidiurnal 

 tides in the North Atlantic are produced this way. 



While the tides in the deep oceans ebb and flow in accordance 

 with the changing positions of the Moon and Sun, the tides in 

 coastal seas, such as the English Channel and the Red Sea, are 

 secondary effects and depend on the oceanic tides. If these seas were 

 cut off from the oceans, the tides in them would be almost imper- 

 ceptible; it is the tidal waves coming in from the ocean that set 

 them oscillating. But here too resonance plays an important part 

 and is responsible for large ranges of tides, as in the Bay of Fundy, 

 Bristol Charmel, and the Sea of Okhotsk. Wherever we find a large 

 vertical rise and fall of tidal water we know that enormous quanti- 

 ties of sea are also flowing horizontally from one place to another; 

 in the Bay of Fundy tidal currents twice daily move 100,000,000,000 

 tons of water. In restricted passages and straits these tidal currents 

 sometimes attain speeds of more than ten knots, as they do in 

 Discovery Passage and in Seymour Narrows, British Columbia. In 

 coastal seas tidal currents frequently attain speeds of two or three 

 knots, but in the open oceans they are rarely more than a quarter of a 



Mont St. Micfiel, tlie fortified abbey off 

 Brittany, at tow tide is surrounded by 

 mites of sand flats. At t]igh tide the sea 

 fioods ttie surrounding plain, leaving the 

 fortress an island linked to the mainland 

 only by a causeway. 



