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I l6 HARBOUR ENGINEERING.
Mediterranean Sea, from 15 to 20 feet ; in the Bay of Biscay, from 25 to 30
feet ; in the open Atlantic, from 30 to 40 feet; and in the Pacific (off Cape
Horn and the Cape of Good Hope) from 50 to 60 feet. Other estimates of a
much higher nature have been made, but it is open to question whether they
have not been influenced by an uneonscious tendency to exaggeration on the
part of the observer, due to the inspiring nature of the spectacle, or been
founded upon mistaken and erroneous data. There is, in such cases, a strong
and an acknowledged inducement to use picturesque language and to speak of
waves as “ mountains high,” to which, of course, numerical values must as far
as possible correspond. Indeed, viewed at close quarters, a formidable wall
of water towering suddenly above the spectator’s line of sight, even when
on the upper deck of a vessel, can hardly fail to produce an illusory sense
of enormous magnitude and overwhelming menace. So far, however, as
unquestionable records go, it may safely be asserted that 50 feet is about the
maximum height attainable by unbroken waves, and this view is supported
by the opinions and testimony of Sir George Airy, Captain Scoresby, and
other observers.
The heights of waves breaking against the cliffs and headlands of a rocky
coast do not, of course, corne within this category. The summits of columns
of water thrown up by the force of impact attain, as might be expected, to
much greater altitudes. The effect is particularly noticeable in the case of
lighthouses and prominences with vertical or nearly vertical faces. Thus, at
The Hague heights of 75 feet, at Bell Rock 100 feet, and at Eddystone
150 feet, have frequently been recorded, and Lord Dunraven has observed
heights of 150 feet on the precipitous South West Coast of Ireland.
« /O 1030 40 50 60 70 BO BO 700
I _L-LJ _I _I 11 I 1___1
200
Scale of Feel:.
Fig. 97. —Section of Holyhead Breakwater.
On the basis of the evident connection existing between the development
of waves and the generating distance, Stevenson, the eminent harbour
engineer, devised an empirical formula to determine the height of waves from
the Fetch, or extent of sea available for the purpose of generation. Taking H