_J I I 



In 1922 Professor J. Proudman and 

 Dr. A. T. Doodson collated many 

 observations of tidal streams from 

 a network of stations in the North 

 Sea. The observations were made 

 with the help of the International 

 Council for the Exploration of the 

 Sea and augmented by the Hydro- 

 graphic Department of the Admiralty. 

 Some of the tidal elevations in the 

 open sea were measured by submarines. 

 The dotted lines show the tidal 

 range at intervals of 2 feet. The 

 solid lines are cotidal lines and 

 join places which have high water 

 sim-ultaneously at successive lunar 

 hours. It is a complicated picture 

 with several amphidromic points. 



Actual tides in the oceans can be 

 regarded as water swinging back and 

 forth in a dish. A small force 

 repeated at just the right timing 

 will maintain a large oscillation. 

 Ocean tides behave much as though 

 the ocean had divided itself into a 

 system of basins with natural 

 periods of oscillation of about 12 

 or 24 hours. The oscillations are 

 not simple movements from end-to-end 

 or side-to-side because the 

 geostrophic effect of the Earth's 

 rotation introduces rotary movement. 

 The figure here shows what is likely 

 to happen in a large rectangular 

 inlet. Without the effect of 

 gyration the water would oscillate 

 from end-to-end about a nodal line 

 as in A and C. But at the inter- 

 mediate times, as water is flowing 

 from the high end to the low end, 

 the gyration makes it move to the 

 right in the Northern Hemisphere and 

 the right hand side is higher than 

 the other. The nodal line becomes a 

 nodal point, called the amphidromic 

 point, and the tide swings around it 

 in a counterclockwise direction. 





Amphidronic 

 , poifit 



263 



