128 A TEXTBOOK OF OCEANOGRAPHY 



and Wales, and on the west by the east coast of Ireland. In 

 this area the whole body of water is moving southwards while 

 the tide is falling at Liverpool north-west lightship, and north- 

 wards while the tide is rising there. 



There is another oscillating area in the north part of the 

 Irish Sea contiguous to the above. The waters in thèse two 

 areas move towards each other while the tide is rising at the 

 Liverpool north-west ship (or Dover), and away from each 

 other with the falling tide there. Summing up, it may be said 

 that " in order to supply the water necessary for the tidal wave 

 as it advances from océan depths toward shallow banks, hori- 

 zontal movements are set up, which oscillate backwards and 

 forwards over certain areas ; and thèse horizontal movements, 

 when obstructed, either by meeting a coast-line or by meeting 

 each other, pile up the waters to a height far exceeding the, 

 height of the normal tidal undulation propagated in the océan, 

 and so produce the abnormal rises and falls which are so noted 

 in the Bristol Channel, at Liverpool, and among the Channel 

 Islands." 



Océan Waves. 



Océan waves are caused by the wind. The term " swell " 

 is applied to waves not produced by the wind in the locality 

 where such waves are met with, but caused by storms at a 

 distance. Occasionally it is possible for an observer on board 

 ship to distinguish " sea " — i.e., waves produced by wind 

 locally — and " swell," waves produced by wind at a distance. 



Measurements of the dimension of oceanic waves are by no 

 means easy, as the sources of error are hard to eliminate. 

 Most estimâtes of oceanic wave-heights recorded by seafarers 

 are too high. In the figure the height given by the observer 

 is cd, whereas the height of the wave is really only ab. The 

 observer on a roUing or pitching ship is the victim of an 

 optical illusion. 



Waves vary in différent local ities, according to the velocity 

 and direction of the wind. The longest wave recorded is one 

 of 2,600 feet length and 23 seconds period by the French 



