THE TIDES 127 



The Tidal Streams. 



So long as the tidal undulation (ravels in great depths it is 

 a simple wave, but when it meets with obstructions it is trans- 

 lated into a wave of horizontal force i.e., a tidal stream, as 

 distinct from a wave, is produced. 



Even off the banks to the south-west of the British Isles, 

 such as the Jones, Scot, and Nymphe Banks, ripples and 

 overfalls are observed even in the finest weather. These are 

 due to the tide. 



These phenomena are universally met with. Where sub- 

 marine elevations are met with, rising suddenly from great 

 oceanic depths (2,000 fathoms), tide ripples are seen even when 

 such elevations are 800 fathoms below the surface. 



The details of the force and direction of the tidal streams 

 round the British Isles are too complicated to be dealt with 

 here.* 



There is, however, one phenomenon which must be 

 noticed briefly. Over certain areas the whole body of water 

 oscillates backwards and forwards with the regularity of a 

 pendulum having a stroke of about 6J hours, the total 

 oscillation being from about 10 to 20 miles backwards and 

 forwards. 



These oscillations occur simultaneously, or nearly so, over 

 areas widely removed from each other, and appear to coincide 

 almost exactly with the rise and fall of the tide in a given spot. 



Fifteen such oscillations are described and figured in the 

 Admiralty manual on the tidal streams of the British Isles. 



One may be quoted as an example. In the southern part 

 of the Irish Sea there is an oscillating area bounded on the 

 south by a line joining Carnsore Point and the Smalls Light- 

 house, on the north by a line joining St. John's Point 

 (Ireland), the Point of Ayre (Isle of Man), and the north end 

 of Walney Island, on the east by the west coast of England 



* See "The Tides and Tidal Streams of the British Isles," etc. First 

 edition, 1909. London : Hydrographic Office, Admiralty. 



