MR. G. I. TAYLOR ON TIDAL FRICTION IN THE IRISH SEA. 25 



velocity of the tidal wave. On examining the data at our disposal it will be found 

 that they are hardly sufficient to place the co-tidal lines for 6, 7, 8, 9 and 

 1 hours sufficiently accurately on the map to make an accurate determination of the 

 velocity of the co-tidal line in the neighbourhood of the line AB. 



On looking at maps of co-tidal lines for the Irish Sea, however, such as that given 

 in KRUMMEL'S ' Ozeanographie,'* it will be seen that the co-tidal lines for successive 

 hours are crowded together in the neighbourhood- of the Arklow-Bardsey line. V c 

 is therefore a minimum in that region, as we should expect from (42). 



Though we cannot measure accurately the velocity of the co-tidal line as it passes 

 the Arklow-Bardsey section, there are two sections of the channel where the positions 

 of single co-tidal lines can be determined with considerable accuracy. We can 

 therefore determine the mean velocity of the co-tidal line between these two sections 

 and can compare this with theory. The line AB, which is practically a co-tidal line for 

 8h. 10m. is one example. Bardsey Island at its eastern end is separated from the 

 mainland, so that the error due to a shelving shore is lessened. The time of H. W. at 

 Courtown, a few miles south of the point where the western end of AB strikes the 

 Irish coast, is 8h., while the time of H.W. at Arklow Bank and Arklow, both a little 

 north of AB, and therefore a little later in their tides, is 8h. 25m. Greenwich mean 

 time. 



The co-tidal line for 8h. 10m. is therefore well determined and is practically 

 coincident with AB. As before we shall take it as being coincident with .* = 0. 



The other co-tidal line referred to is the one for Gh. 15m. It is H.W. at 

 Carnsore Point at 6h. 25m., and at Tuskar Rock, 4f miles off the Trisli coast, at 

 6h. 10m., while the other end of the line is determined by Ramsey Sound, off the 

 Welsh coast, where it is H.W. at 6h. 21m. The co-tidal line for 6h. 15m. is shown as 

 the line TS in the map, fig. (3). 



Let us then apply the formula (4l) to find the ratio of the amplitudes of the two 

 tidal waves. The distance between the mid points, M and L of the two co-tidal lines 

 AB and TS, is about 43 nautical miles, so that in (41) .< = 43 miles. The mean 

 depth of the water between the two sections is 45 fathoms, and the velocity of a long 

 wave in water of this depth is 56 nautical miles per hour. Remembering that T, the 

 period of the semi-diurnal tide is 12h. 25m. or 12'4h., it will be found that 



Also t x = Gh. 15m. - 8h. 10m. = - l'92h. and 



-56) = -0-67 ..... (44) 



* ' Handbuch der Ozeanographie,' vol. 2, p. 336 (1911 edition). 

 VOL. CCXX. A. E 



