-MB. G. 1. TAYLOR ON TIDAL FRICTION IN THE IRISH SEA. 
0 
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 
= Hcosy(^ + 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 tlie time of the moon’s passage over the Greenwich 
meridian. At full and change of the moon, t is Greenwich mean time. 
Tj 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 on 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; Ail^low Bank, feet; and Kilmichael Point, 
4f feet; while on the Welsh coast there are St. Tudwall Road, 14 feet; Port Dynllayn, 
12^ feet; Llanddwyn Island, 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 difierence 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. 
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