MR. G. I. TAYLOR ON TIDAL FRICTION IN THE IRISH SEA. 9 



large majority of cases this can be represented with sufficient accuracy for most 

 purposes by a term of the form h, the height of the tide above mean sea-level 



^+T,) (16) 



where 



2H is the range of tide between H. W. and L. W. 



T is the tidal period of 12h. 25m. 



t is the time measured from the time of the moon's passage over the Greenwich 



meridian. At full and change of the moon, t is Greenwich mean time. 

 T, is the time of H.W. at full and change of the moon, i.e., the " establishment" of 



the place in question. We shall henceforth assume that h can be expressed 



by means of the equation (16). 



In evaluating the integral (15) it will be seen that it is necessary to know the 

 height of the tide at all points on the section. Unfortunately nearly all the 

 measurements of rise and fall of tide have been made on the coast. None have been 

 made in the middle of the channel, or at any rate none are recorded in the 

 tables. 



At first sight we might expect tidal range to be the same on the two sides of a 

 channel, but this not the case. On the opposite sides of the South Channel, at the 

 entrance to the Irish Sea, for instance, the tidal ranges at spring tides are 4 feet at 

 Arklow on the Irish side and 15 feet at Bardsey Island 011 the Welsh side. This 

 is not an accidental circumstance connected with particular formations of the coast 

 in the neighbourhood of Bardsey or Arklow ; all the tidal ranges in the neighbour- 

 hood show the same characteristic. On the Irish coast there is Arklow with a tidal 

 range of 4 feet ; Courtown, 3f feet ; Arklow Bank, 4 feet ; and Kilmichael Point, 

 4f feet ; while on the Welsh coast there are St. Tudwall Road, 14 feet ; Port Dynllayn, 

 12 ^ feet; Llancldwyn Island, 14^- feet; Bardsey Island, 15 feet; and Holyhead, 

 16 feet. In evaluating the integral (15), therefore, it is important to know how 

 the tidal range varies from the Welsh to the Irish coasts. In other words, does the 

 level change more rapidly near the Welsh or near the Irish coast, or does the sea 

 at H.W. slope uniformly down, and at L.W. slope uniformly up, from Bardsey to 

 Arklow ? In deciding this question, dynamical considerations are of great 

 assistance. 



The reason for the difference in the range on the two sides of a channel is well 

 known ; it is connected with the " geostrophic " force, due to the earth's rotation, 

 which tends to deflect bodies moving on the earth's surface to the right in the 

 Northern, and to the left in the Southern Hemisphere. The flood stream into the 

 Irish Sea cannot be deflected to the right because of the Welsh coast. The water 

 therefore piles up on that side till the hydrostatic pressure-gradient is sufficient to 



VOL. ccxx. A. c 



