MR. G. I. TAYLOR ON TIDAL FRICTION IN THE IRISH SEA. 27 



involves only measurements of depth and time of H.W. It does not involve 

 measurements of current or tidal range at all. 



The remarkable agreement between these two methods of estimating the loss of 

 energy in the tidal wave during its passage into and out of the Irish Sea is strong 

 evidence that three-quarters of the energy of the tidal wave entering by the South 

 Channel is dissipated in the Irish Sea. The main purpose of this paper is therefore 

 accomplished. 



To complete the investigation, however, it is worth while to show that certain 

 other tidal phenomena of the South Channel, hitherto apparently not very well 

 understood, are simple consequences of the superposition of two tidal waves of 

 different amplitudes moving in opposite directions. 



The first of these is the difference between the velocities of the co-tidal line on the 

 two sides of the channel. This difference is very marked. The distance from Tuskar 

 Rock to the south end of Arklovv B;mk, traversed by the co-tidal line up the Irish 

 Coast during the interval between 6h. 15m. and 8h. 10m. is only 30 nautical miles. 

 while the distance from Ramsey Island to Bardsey Island traversed by the co-tidal 

 line in the same interval on the Welsh ('oast is 59 miles, so that the velocity of the 

 co-tidal line on one side of the channel is about double its velocity on the other. 

 This difference evidently causes the co-tidal line to turn through a large angle. 

 independently of any turning which the wave fronts themselves may experience m 

 passing through the channel owing to a difference between the depths on the two 

 sides. 



In the case of the South Channel the depths are practically equal on the two sides, 

 so the fronts of the waves will'not turn, though of course they may be very slightly 

 convex, owing to the greater depth in mid-channel. 



The direction of the line AB from South Arklow to Bardsey is N. 8(5 W. The 

 direction of the line TS from Tuskar to Ramsey is S. 57^ W. The angle turned 

 through by the co-tidal line from 6h. 15m. to 8li. 10m. is therefore N. 8fi W. minus 

 S. 57 W. = 36|. 



As a matter of fact the angle turned through by the co-tidal line is. if anything, 

 rather greater than this, because the true co-tidal line for Sh. 10m. must be a little 

 north of Bardsey and south of South Arklow, while the true co-tidal line for 6h. 15m. 

 must run from a point slightly north of Tuskar Rock to a point slightly south of 

 Ramsey Island. The true angle between the co-tidal lines for 6h. 15m. and 8h. 10m. 

 is therefore slightly greater than 36|-. 



Now let us turn to the explanation. It has been pointed out already (see p. 9) 

 that the " geostrophic " or deflecting force due to the earth's rotation increases the 

 rise and fall of tide on the side of the channel which lies on the right-hand side of an 

 observer who faces in the direction in which the tidal stream is running at H.W. In 

 the case of a progressive tidal wave the current at high water is moving in the 

 direction in which the wave is travelling. 



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