A sixteenth-century circular tide table. 



Ports located in areas having a tidal range 

 of fifteen feet or more require a system of 

 closed docl<s. The water level Is kept 

 more or less constant by one or more 

 locks. The Royal Docks, Woolwich, part of the 

 great dock area of London, are shown here. 



knot. Wherever tidal currents are strong, there is danger for ships. 

 One of the more notorious tidal current areas of the world is around 

 the Aleutians, where ships have been cast up on the rocks as they 

 attempted to work their way through the Akutan Pass and the 

 Unalga Pass. 



Most of the great ports of the world are situated in tidal waters. 

 The tidal regime not only affects their everyday working but 

 determines the character of the port's layout and installations. 

 Where the range of tide is not more than about fifteen feet, the 

 open quay system can be used, as in New York and Southampton, 

 but the depth of water at the quayside must be great enough to 

 keep the ships afloat at low water. 



Where the tidal range is greater than fifteen feet, port engineers 

 have to plan an elaborate system of closed docks like those at 

 Liverpool and London. A ship passes through a lock and into the 

 dock basin, where the water is kept at a more or less constant level. 

 When the approach channel to a port is not deep enough at low 

 tide for large ships of deep draft, the ships' movements to and from 

 the port are dependent entirely on the tides. The importance of 

 accurate tidal predictions - of heights as well as the times of high 

 and low water — is obvious in such cases. 



Today we have tide-predicting machines that can quickly work 

 out the times of high water and low water at a given place for any 

 future date. The basic information needed is a continuous set of 

 observations — preferably over a year — of the tides at the place in 

 question. This information is analyzed and the results used together 

 with our knowledge of the positions of the Sun and Moon at any 

 given moment to predict future tides. 



To a ship's pilot and to a navigator, a knowledge of tidal currents 

 is as important as that of the tidal rise and fall. Before a pilot brings 



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