When
obstruction occurs, as, for example, in the Bristol Channel,
where there is a gradually rising bed with a converging
channel, the velocity, and/or the amount of rise and fall of
the derivative wave is increased to an enormous extent; in
other places where the oceans widen out, the rise and/or
velocity is diminished, and similarly where a narrow channel
occurs between two pieces of land an increase in the velocity
of the wave will take place, forming a race in that locality.
Although the laws governing the production of tides are well
understood, the irregularities in the depths of the oceans and
the outlines of the coast, the geographical distribution of the
water over the face of the globe and the position and declivity
of the shores greatly modify the movements of the tides and
give rise to so many complications that no general formulae can
be used to give the time or height of the tides at any place by
calculation alone. The average rate of travel and the course of
the flood tide of the derivative waves around the shores of
Great Britain are as follows:--150 miles per hour from Land's
End to Lundy Island; 90 miles per hour from Lundy to St.
David's Head; 22 miles per hour from St. David's Head to Holy
head; 45-1/2 miles per hour from Holyhead to Solway Firth; 194
miles per hour from the North of Ireland to the North of
Scotland; 52 miles per hour from the North of Scotland to the
Wash; 20 miles per hour from the Wash to Yarmouth; 10 miles per
hour from Yarmouth to Harwich.
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