12 ME. G. I. TAYLOE ON TIDAL FEICTION IN THE IRISH SEA. 



The considerations just advanced show that if a channel is so narrow that the 

 water is forced to travel straight up and down it. then the difference in level between 

 the water on the two sides may be calculated on the assumption that the sea slopes 

 to an extent which gives rise to a pressure gradient across the channel which is 

 exactly equal to the deflecting force due to the earth's rotation. If the channel is 

 rather wider the central parts of the stream may be able to move across the channel 

 slightly. This would reduce the slope. In the case of the South Channel of the 

 Irish Sea, however, these cross currents are very small, as may be seen by examining 

 the figures given in the table on p. 14, where it is shown that the direction of the 

 current is practically constant during the ebb and during the flood streams. We are 

 therefore justified in assuming that the South Channel is narrow enough to allow us 

 to apply the calculations given above. 



Velocity of the Tidal Currents. -We now come to the measurements of tidal 

 currents. These are the principal factors which determine our choice of sections 

 suitable for measuring the flow of energy into the Irish Sea. 



South Channel. In the South Channel the best section is that shown as AB in the 

 map (fig. 3). It runs from Bardsey Island through the south end of Arklow Bank. 

 Along this section tidal measurements have been made at the points marked in the 

 map as S u S 2 , S 3 , S 4 . 



Tn the position Sj, 5 miles from Arklow Bank, the maximum velocities of the ebb 

 and flood streams are both 3'6 knots. The direction of the flood stream is N. 32 E., 

 while that of the ebb is S. 26 W. 



At S 2 , 15 miles from Arklow Bank, the maximum flood stream is N. 35 E. at 

 3 '2 knots, while the maximum ebb stream is S. 32 W. at 3 '3 knots. 



At S 3 , 15 miles from Bardsey Island, the maximum flood stream is N. 25 E., 

 3 '2 knots, while the maximum ebb is S. 28 W., 3'0 knots. 



At S 4 , 5 miles from Bardsey Island, the maximum flood stream is N. 16 E., 

 3'0 knots, while the maximum ebb is S. 16 W., 2 '3 knots. 



It will be seen, therefore, that the maximum current velocity is nearly constant 

 along the section, its average value being 3 '2 knots. The direction also varies very 

 little ; the average direction of the flood stream being N. 27 E., while that of the 

 ebb S. 26 W. These are practically opposite directions. They will (for simplicity) 

 be assumed to be exactly opposite during the rest of this discussion. 



No measurements of the speed and direction of the currents at the points 

 Sj, S 2 , S 3 , S 4 , are given for the intermediate hours ; but several such measure- 

 ments are given for other points in the neighbourhood. I have selected three 

 of these sets which were taken at the nearest points to the section AB. They 

 were made at the South Arklow, North Arklow and Carnarvon Bay light-ships, 

 respectively. 



In the accompanying table* are given the velocities and directions of the current 

 * Taken from ' Tides and Tidal Streams of the British Islands,' first edition, 1909. 



