his ship into or out of port he must know exactly how the tidal 

 current is flowing. Large ships such as the United States must wait 

 for slack water before attempting to tie up at their piers in New 

 York; if they did not, the strong tidal current could swing them 

 against the pier with crushing force. Charts of tidal currents are, 

 therefore, as indispensable to the mariner as his tide tables are. 



It is not only those who have a professional interest in the sea 

 who find their activities dependent on the tides. In many seaside 

 resorts the holidaymaker soon learns that he has to arrange his 

 bathing, sailing, fishing, or building of sand castles according to 

 the tide. 



When tides flow into an estuary they become distorted and 

 slowed down by the restricting effect of shallow water. What usually 

 happens is that the water level rises more rapidly than it falls, and 

 the flood stream flows faster than the ebb, although for a shorter 

 length of time. In extreme cases the flood stream rushes up the 

 river in the form, of a tidal bore — a turbulent mass of water with 

 an almost vertical wave front, followed by a series of choppy waves. 

 There are some half dozen or more well-known tidal bores in the 

 world. In the British Isles, bores occur in the River Trent, a trib- 

 utary of the Humber, in the River Severn, and in the Solway 

 Firth. One of the most striking bores is in the Chien-Tang Kiang, 



The map on the following pages shows tidal 

 systems of the main oceans (based on 

 Dietrich, 1944). The cotidal lines - lines 

 joining places which have high tides 

 simultaneously - are based on semidiurnal 

 tides due to the Moon and do not include 

 the Sun. Numbers on the lines indicate 

 the time of high water, in "lunar hours," 

 after the Moon has crossed the prime 

 meridian. Every twelve lunar hours there 

 is one tidal period (equal to 12 hours, 

 25 minutes of mean sun time). The cotidal 

 lines shown on the map are for the oceans 

 only; cotidal lines for the enclosed and 

 peripheral seas are too complex to be shown 

 on a small scale map. 



This nineteenth-century drawing shows a tidal 

 bore rushing up the Chien-Tang Kiang, a river 

 that empties into the Bay of Hangchow. The 

 wall of tidal water forming the bore reaches 

 a height of about eleven feet and moves up 

 the river at a speed of about twelve knots. 

 There are a half dozen or more well-known 

 tidal bores in the world. 



