PNEUMATICS AND HYDRAULICS.—POWER OF THE WIND.—POWER OF THE
WAVES, AND VELOCITY ACQUIRED BY THE WATER.—OCCASIONAL HEIGHT
OF TIDES.—TIDAL WAVE.—CURRENTS.—TIDAL PHENOMENA ON THE ISLE OF
WIGHT.—VARIATIONS IN THE COAST-LINE.
The elemental powers which are constantly at work in Nature all around
us are upon a vast scale; a scale, in fact, which is immeasurable by
us, except when observed under certain favourable circumstances. As our
practical ideas of locomotion are immensely surpassed by the speed of the
Earth’s rotation on her axis, which is at the equator rather more than
1000 miles an hour, and yet far more surpassed by the rate at which she
moves in her orbit round the Sun, which exceeds 20,000 miles an hour, so
also our experience of a puff of wind on the hillside, or a dash of water
in our face, when a slanting shower or the spray from a cataract salutes
us, gives us no conception of the stupendous powers which are exerted
when the sea, convulsed by a storm, rages along some unprotected coast.
Once in my life, I remember being knocked down by a blast of wind; and
several times, as I suppose has happened to many persons, I have been
_floored_ by an unexpected billow while bathing. I always made this
reflection at the moment: “Its power is not known: it has floored _me_
easily enough, but perhaps it would have felled an ox.”
But a wave impelled by the gale, which could take an ox off his legs,
is nothing at all. What is the measure of force actually applied when a
stout ship is shivered on a lee shore? No one knows. When the Clarendon,
West-Indiaman, struck on that fatal “race” off Blackgang Chine, she went
utterly to pieces _within seven minutes_! Yet, what a delicate thing is
AIR! What a yielding thing is WATER! But then the air and water threw her
_on a lee shore_.
Observations, carefully made at the right time and place, enlighten
our ignorance upon all these matters. Air, delicate as it seems, when
compressed, explodes in the roar of the thunder; and water is almost
_in_compressible, and therefore its blow will knock down _anything_.
Let us speak first of the power of the WIND. The sands of the Desert, as
has been said, are powdered quartz and quartz is a heavy substance; but
when a strong wind ploughs the surface of the Desert at an angle, these
Sands are lifted, and made to gyrate in spiral folds, and a huge column
is formed—perhaps a hundred and fifty feet in height—and this monstrous
trunk is presently carried along at whirlwind speed; and if it meet a
party of travellers, they will be overwhelmed and buried.
It may be said that the sand is in very minute particles; and this is
true. But if they are, therefore, the more easy to disturb and to catch
up aloft, they are all the more difficult to bind together in a spiral
form, and to hurry along the desert unbroken.
The power of the wind, especially when it comes in sudden gusts, and with
a whirling motion, against timber-trees and the upper parts of buildings,
is only too well known; on wide, exposed plains, its fury, in the early
part of the year, sometimes brings destruction to everything within its
range. But it is on the open sea, where the hurricane, once let loose
from heaven, can sweep, unchecked, perhaps for hundreds of miles, that
the latent forces of this unseen element are revealed in all their terror
and majesty. For here the wind not only has free scope, but it also finds
another element, fluid but incompressible, to obey its impulse and follow
in the course which it takes. If the WIND be like a wild spirit, the
WATER is a mighty, irresistible body, endowed with motion by the other,
and capable of any work, from the drowning of a sea-gull to the wrecking
of a stately ship.
The waves which roll in from the open sea, when the tide is _making_, are
the most powerful. Fortunately, these do not reach the shore with quite
the same force which they exert at the distance of some miles from it;
but in exposed situations, their altitude and momentum are very great.
The billows of the Atlantic which break on the western coast of Ireland
run from thirty feet to fifty feet in height, and they arrive on that
coast with an impetus which has (up to a certain point) been gaining
strength, perhaps for half the time of a tide. It was this fearful onset,
from a foe that never slumbered, which broke up and rendered useless the
eastern extremity of the great Submarine Electric Telegraph Cable between
Ireland and America.
It has now been decided that when stormy winds prevail long in one
direction, another and peculiar force is given to the waves on which
they operate. For the wind, by pressing long _on the side_ of a wave,
changes its form from that of an upright ridge (_i.e._ with _vertical_
axis) to one which has a stoop or bend, sometimes of great inclination.
Such a wave will come in upon the shore with far greater momentum, for
its _velocity_ has been enormously increased, while its _bulk_ is nowise
diminished. The speed which the water acquires under the influence of a
prolonged sou’-wester on the coasts of Sussex and Hampshire, although
there the gale has only traversed the breadth of the Channel, is very
notable at times. At Lowestoft and at Peterhead, easterly points of land,
the danger of the winds from the North Sea is proportionally far greater,
as a larger body of water has been continuously acted upon. For it must
always be remembered that a wave at sea is simply an _oscillation_ of
the water which rises and falls in that place. The identical wave does
not pass on to the shore, though it appears to do so; it is the _motion_
which is propagated, and, as it were, handed on, like an electric shock
through the successive plates of a battery. Now this motion _increases_
with the onward career of the gale. “Vires acquirit eundo.” And when the
mass of the fluid over which the wind is blowing has once been agitated,
and the equilibrium of its upper layers been thoroughly disturbed, if the
current of air not only continue, but wax stronger and stronger, there
scarcely seems any limit to the momentum which the rolling wave may thus
acquire. The arithmetic of a few facts upon this subject will, however,
give us some idea of what that momentum must be on certain occasions.
It has been found, by experiment, that the velocity imparted to an
incoming wave sometimes equals _seventy feet in a second of time_. Now
this would give nearly one mile in a minute, if it were a projectile in
free space; but the case here is, of course, different, since the wave
is only a portion of the mass to which it belongs, and no individual
wave travels very far. Still, the amount of rush and pressure which are
exerted is altogether stupendous. Probably the _breakers_, which in an
enduring storm rise against a lighthouse in the open sea, are among the
strongest instances with which we are acquainted, and, next to these, I
suppose, the billows off the Cape of Good Hope. But we may take those on
the Irish coast, westward, as a well-known sample. I have often watched
these—once in a gale of wind—and I have seen, in one as yet _unbroken_
wave, a body of water which I should compute roughly at two hundred tons’
weight, and as having, at the moment, a velocity of not less than sixty
feet in the second, discharge itself upon the reef of rock. On such
occasions, if you happen to stand on the shore, you will feel the ground
_vibrate_ apparently, though the sensation is probably electrical. But
let any one consider the conditions of the wave cited above, and reckon
what is _the force of the blow_ at the moment of impact. I have no doubt
it would knock down a good strong house, if delivered against the upright
face of it.
One winter, when I was at Sandown, in the Isle of Wight, a spot
frequently mentioned in these pages, I found I was _utterly unable_
to traverse the bit of open road reaching from the Fort to the point
where the lane turns off to Yaverland. Yet I had on at the time a stout
Cording’s waterproof, with sleeves, and a slouched hat tied under my
chin, and held a powerful oaken stick in my hand to help me win my way.
In fact, it was a trial of dynamics between me and the gale, whether
this “body,” acted on by a continuous muscular force, in nearly a right
line, could be propelled through that resisting medium. I am sorry to
say, it could _not_; but, in the attempt, it lost a neck-button from its
coat, and was within an ace of being blown into the hedge, like a certain
“man in Thessaly, wondrous wise,” whom we have heard tell of in our
childhood.
This storm held on for seven or eight hours, and as the sea rose many
feet higher than usual under its influence, when the tide was well
in, the strife of elements, and their combined assault upon the works
of men’s hands were truly grand. A breach was made through the solid
causeway, compact long ago of clay, and gravel, and boulders of flint,
and guarded seaward by dykes of timber. This breach was effected, I have
no doubt, by the _stroke_ of the wave, as the sword of Roland is said
to have cut through the rock in the fight of Roncesvalles. A wall of
stout masonry, not above five feet in height, and supported behind by
earthworks, so as to resemble an escarpment, yielded before the weight
of water, as a pane of plate-glass would give way at the charge of a
locomotive engine. Some twenty yards’ length of it was rent and thrown
down. On examining this fragment afterwards, I found that the materials
of the wall were solid blocks and angular bits of Wenlock limestone, and
the cement used of the strongest. I believe, however, if it had been
three times as strong a barrier, those waves would have levelled it.
I subsequently witnessed in that very neighbourhood a yet more furious
tempest of wind, but had no opportunity of measuring its effects.
At Eastbourne, about two years age, I made acquaintance with a phenomenon
in this line which was altogether new to my experience. We had a
tremendous gale late in the autumn of 1857. I cannot exactly cite the
quarter from which the wind blew, as I do not accurately know the points
of the compass in their bearing on the town and bay; but the storm seemed
to beat in from the south-east, as it faced the Marine Esplanade. I went
forth, with perhaps a score of others, to gaze upon this magnificent
“encountering shock” of earth, air, and water. We were all of us drenched
to the skin by the dashing spray, and occasionally well-nigh swept off
our legs by the gust; but we held on stoutly, _till_ something saluted
those nearest the beach, which rendered a retreat imperative. This was
not the salt spray, nor the rattling hail neither, but a cloud of
“skirmishers” in the shape of _pebbles and gravel_ from the strand below.
The sea had actually lifted the surface of the bed of beach, and whirled
aloft some bushels of its solid contents. A coast-guard told me next day,
that he saw a “flight” of pebbles, some of them as big as hens’ eggs, at
least thirty feet high in the air; and he added, that if the direction
of the wind had changed a little, every pane of glass on the ground and
first-floor of the Esplanade must have been shivered to atoms. As it was,
of course much damage was done to windows and sashes; but not by the
actual pebbles.
After this gale had subsided, I ascertained by personal inspection that
some hundred tons’ weight of solid shingle had been moved along the shore
a distance of a quarter of a mile, in the space of a few hours. This was
by the sidelong _drag_ of the tide.
The village of Seaford, on this same line of coast, had long been in
great danger of being swept away by the tide. Its outworks and very
standing ground were perceptibly yielding, season after season.
About eight years ago, the inhabitants took the alarm, and drew up a
remonstrance and petition, which were duly forwarded to the authorities.
Proper officers were deputed to go down from London and report; and
the result of their representations was that an able engineer from
head-quarters was empowered “videre ne quid Respublica detrimenti
capiat;” or, in the modern vernacular, to see that the Queen’s lieges in
Seaford suffered no wrong.
The remedy hit upon was simple and forcible; but, assuredly, it was an
instance of what is called “Hobson’s choice.” The engineer said, “There
is only one thing to be done; a solid breakwater must be formed west
of the town, to break the rush of the sea and divert the course of the
current. We will throw down a section of the chalk-cliff; that will
make a mound, on which works of masonry, if necessary, can be erected
afterwards.”
This was done about two miles east of Newhaven, by firing an enormous
charge of gunpowder in chambers drilled in the limestone. I went over
from Brighton to witness it, and the sight was a striking one, as soon
as the smoke and dust consequent on the explosion had cleared away. The
operation, neatly conducted with an electric battery, proved successful;
and its result, in the amount of chalk thrown down on the beach, was
judged sufficient for the time.
But for this “pièce de resistance,” Seaford might by this be in a fair
way to furnish to future generations a gigantic specimen of a marine
fossil.
The danger to Seaford arose not from storms or any casual visitations,
but from the steady continuous action of the tide encroaching on the
line of coast where the town stands. Whether the chalky breakwater above
described will long suffice to counteract this elemental mischief may be
doubted. If after a while it should prove inadequate, the operation will
have to be repeated on a larger scale.
The height of the tides at sea is always known; and it varies little,
depending upon astronomical causes, chiefly on the attraction exercised
by the moon. But on shore, and inland in certain rivers, the case is
widely different. Here the land itself, with its rocks and embankments,
introduces artificial conditions which influence the local tides in an
extraordinary manner. Thus, at Chepstow, by the Castle rock, the rise
of a spring-tide is sometimes as much as _sixty feet perpendicular_. In
Mount’s Bay, Cornwall, it is fully _forty feet_; while in the north of
Scotland on the coast, it does not average more than _ten_.
The tides have other variations beside this of local magnitude; and one
of these, which takes place along our southern coast, is interesting and
important, but was little known till the Admiralty surveyors went to
work. It may be briefly described as follows.
As far as Scilly Isles and the Lizard’s Point, the great tidal wave,
which twice in every twenty-four hours flows in from the Atlantic,
maintains pretty nearly the same level on the opposite shores; and this
continues as far as Exmouth on the Devon coast, and St. Malo on that of
Brittany. But when this great wave reaches the “Needles” which form the
westerly point of the Isle of Wight, it divides into two very unequal
portions. Of these, the southern and larger portion sweeps round the
base of the island, passing Freshwater and Chale and Blackgang, and
then rounding St. Catherine’s promontory and running up to Dunnose with
scarcely-diminished velocity; but the northern and smaller half enters
the Solent, and owing, it is supposed, to the resistance of two shores
(by _friction_) to its progress, advances but slowly, and does not arrive
at Southampton Water before the other has made Dunnose and is filling
Sandown Bay with a _back_-stream. This causes many differences in the
amount of water at almost any given moment on the two sides of the Isle
of Wight, north and south; and it gives rise to many curious varieties
of tides, real or apparent. In one spot near Shanklin, my attention was
frequently drawn to the fact of a partial tide suddenly _flowing_, for
perhaps the space of an hour, during the time of ebb.
But this is not the only “water-company” here at work, nor is the above
the only class of phenomena resulting from such agency. Beside the
_tide-wave_ there is the Ocean-_current_, which is quite a distinct
thing. This current is due to a branch of the Gulf-stream; and it flows
here from west to east, and from south to north. That is to say, after
leaving Newfoundland, so much of it as actually reaches Britain would run
up St. George’s channel northwards, and up the English channel eastwards.
And although the velocity of this current is but trifling, yet, owing
to its unceasing action in one direction, its effects are remarkable.
Moreover, it is not affected by the waves; whether the sea be sleeping in
a calm or tossed with storms, the motion of the current is the same. The
only change as yet observed is when its waters come to the surface and
have their temperature lowered by a chill wind; for, this gulf-stream is
itself a river of _warm water_, though its bottom and banks are of the
same element _cold_.
Thus, between tide-waves and currents, changes are going on continually
both in the volume and the physical constitution of the sea-water.
And this, which might at first on a hasty glance look like casual and
unimportant variation, when we come to consider it attentively, is found
to be an arrangement fraught with wisdom and beneficence; for it insures
continual variations of temperature, it attracts the purifying storms,
restores the elastic spring of the atmosphere, and provides for weary
man a healthy tonic breeze when he rambles along the beach or scales the
face of the cliff. Without the motion imparted by the tides, the sea
would probably become putrid. Now the tides depend upon the MOON; yet
how seldom do we think of her invaluable services, when we gaze upon her
pale face! An Italian once told me that he “loved the moon, adored the
moon, never tired of looking at the moon.” And the Padishah,[4] I have
heard say, prefers “moon-faced” ladies for his soft companions; but I
doubt whether either the pensive Italian or the glittering lord of the
Bosphorus ever bestowed five minutes’ thought on the mighty phenomenon of
the TIDES.
There are other changes in what may be termed the tidal “high-water
mark,” due to the lapse of ages, which the sea has chronicled on the face
of many a shore and cliff. It is well known that the coast-line, in this
and other countries, has experienced many alterations as to its level. In
some places, the ocean has apparently encroached upon the land; in many
others, it has receded. One striking _index_ of this latter, and which
may be implicitly trusted as proving the fact, occurs where the waters
have retired from their former level, and have left exposed to view the
remains of an ancient _beach_. Thus, between Brighton and Ovedean, is
found what goes by the name of the “Elephant-bed,” because such fossil
bones lie among the pebbles in this part of the cliff. The entire stratum
undoubtedly was at one time a beach. Similar phenomena have been traced
along the coast-lines of Arbroath and Cromarty shires.
Elsewhere, the sea has gained upon the land; undermining the friable
sandstone cliffs, and, as it were, melting down the headlands, so as to
change the outline of the coast. But in these latter instances, although
“terra firma” has given way, it may be doubted whether the water itself
has _risen_. Wherever it has indubitably done so, to the extent _e.g._ of
submerging a village, I should apprehend the presence of volcanic agency.
[4] Sultan, “Father of the Faithful.”