56 Dr Hibbert on the Currents of ' Tide of the Pentland Frith . 
of the coast of Ireland. This wave soon afterwards divides it- 
self into three ; one part passing up the British channel, another 
ranging along the west side of Ireland and Scotland, and the 
third entering the Irish channel. The first of these flows 
through the channel at the rate of about 50 miles an hour, so 
as to pass through the Straits of Dover, and to reach the Nore 
about twelve at night. The second being in a more open sea, 
moves with more rapidity ; by six it has reached the north ex- 
tremity of the Irish coast ; about nine it has got to the Orkney 
islands, and forms a ridge or wave extending due north ; at 
twelve, the summit of the same wave extends from the west of 
Buchan eastward to the Naze of Norway ; and in twelve hours 
more it reaches the Nore, where it meets the morning tide, that 
left the north of the channel only eight hours before The 
explanation, then, of the opposition of tides or Roust , as it is 
named at Sumburgh Head, may be given in connection with 
that wave of tide propagated from the great diurnal undulation 
of the Atlantic, which, in the progress of completing its circuit 
round Britain, is described as passing to the west of Orkney, — 
from thence to the north of the British Isles, and then taking a 
southerly direction, so as to form a ridge that extends between 
Buchan and the Naze of Norway. The tides of Shetland ap^ 
pear to be induced by lesser currents, generated during the pro- 
gress of the wave along the westerly, northerly, and easterly 
parts of the country, and these set in nearly an hour sooner on 
the west than on the east coast of these islands. At the begin- 
ning of the flood, the tide in the Roust is directed to the east- 
ward, until it passes the promontory of Sumburgh ; it then meets 
with a south tide, that has been flowing on the east side of the 
country ; a divergement then takes place to the south-east, and 
lastly to the south. At high-water there is a short cessation of 
the tide, called the Still ; then the ebb begins, first setting north- 
west and then north, until the recommencement of the flood. 
The various directions of the tides of Shetland are no doubt 
owing, in a considerable degree, to modifications which take place 
from the number and form of the various headlands and inlets 
of the coast; but since they are propagated at successive inter- 
vals of time, it is evident that at the northerly and southerly 
* Playfair’s Outlines of Natural Philosophy, vol, ii. p. 338. 
