Portland Island – 1858
continued
A
little farther study of the coast shows how the very same winds that drove in
the seas to start the shingle, drive in seas that stop it.
The sharp angle in the direction of the shore – causes the shingle to
sweep round and meet, in an easterly direction, waves that are coming from the
very same quarters as those which originally gave it motion.
One incidental proof that the ultimate movement of shingle along this
coast is decidedly eastward, is well worth mentioning.
Years ago, at the time when flints were in general demand for gun-locks,
and for striking lights in domestic uses, it was the constant custom to send
from Budleigh Salterton to Sidmouth, or Branscombe, to procure them, as none
could be found upon the beach at the former place.
The
extraordinary isolation of the bank from the land for upwards of ten miles is,
probably, due, in part, to the existence of a level beach of clay under the
shingle; in part to the sudden drop seaward.
This clay beach has been found, as yet, on the east or inner side only;
where it lies in certain points at a level of from three to four feet above
low-water of spring tides. At some
distance below the surface, sand is often found mixed with the shingle, and that
to such extent, that it has required a power of many tons to extract, from a
depth of eighteen or twenty feet, a bar of an inch and a quarter in diameter.
The great elevation of the shingle is to be attributed to the unusual
depth of water close beside it, upwards of eight fathoms at the distance of a
cable's length. This surprising
depth it is which allows the heaviest seas, check by no shoal-water in the
offing, to fall on the bank with great violence, and throw up shingle with a
will.
The
force of the sea on the Chesil Bank during a heavy south-west gale is
tremendous. It often happens that
the water receding from any wave just broken, meets that of the wave next in
order, in its progress shoreward. The
concussion is go great that an enormous quantity of broken water and spray will
sometimes rise perpendicularly into the air to a height of sixty or seventy
feet. Meets of this sort have
broken up stranded vessels instantaneously of two hundred tons burthen. And then, what masses of shingle will the sea on fit occasion
scour away! After the gale of
December the twenty-seventh, eighteen hundred and fifty-two, the quantity
scoured off between Abbotsbury and Portland was accurately ascertained to have
been three millions seven hundred and sixty-three thousand, three hundred tons!
By sections taken at the next spring-tides, it was found that – after
the gale, and between the same points - there had been thrown in two millions,
six hundred and seventy-one thousand, five hundred tons.
On November the twenty-third, of the same year, the wind, which had been
light during the day, suddenly freshened to half a gale at four P.M., blowing
south and south-west, and at eight P.M. had almost died away again.
Here was a duration of only four hours, yet, it was sufficient to start a
very heavy groundswell on the bank, which scoured away during the night and
early morning, four millions and a-half of tons. Measurements were taken five days afterwards, and it was
found that three millions and a half had been thrown back again during that
interval!
The
wandering shingle has nevertheless, at Chesil, at least the appearance of rest;
the long line of the isthmus is finished, and the noble curve complete.
It is hard to say too much of the extreme beauty of this curve, and of
the grand view which is to be had of it from the summit of the hill, more
especially when the bank has its western slope fringed with the broad white foam
of a heavy sea, and wears a veil of cloudy spray.
But
now for the Breakwater, of man's devising:
We
land from the steamer about midway between the Breakwater and the shingly
isthmus. Turning to the left from
the end of the small pier, a quarter of a mile of road skirting the beach, and
flanked on the right by the slope of underlying clay which forms the base of
Portland, we come to the entrance gate of the Works.
Names must be entered here in the visitor's book; two melancholy
policemen narrowly eye our method of penmanship and eagerly peruse names and
addresses when our backs are turned. We
walk forward at once towards the huge staging.
The pathway is lined with blocks of stone, iron rails, and timbers; here
and there lies a broken pile, with the shoe and Mitchell's screw attached.
On
our right is the engineer's office, at one end of which lies a magnificent
specimen of the fossil trees that abound in the dirt-bed stratum. This tree-trunk measures almost three feet across; and, when
found, was more than thirty feet in length.
About a hundred yards beyond the entrance gate a broad ladder brings us
up to the staging, or Cage itself, where we at once get a more enlarged notion
of what is going on. A great space,
covered with workshops, lies close by, just under the hill; and, among the
workshops, are large masses of dressed stone, upon which the masons and
stone-cutters are hard at work. Up
the hill to the right run the inclines; the heavy four-wagon trains rattle down
them and flit by us, each with Prince Albert, or Prince Alfred puffing away
behind, and dashing them off rapidly to the far end of the cage.
A mile of this fine stage-work is complete, and one cannot do better than
start off and walk the mile. A good
railed passage is provided, leading between two of the five broad-gauge roads
which run to the end of the inner breakwater abreast over open rafters.
The large blocks of heaped stone, which at first underlie the rafters,
soon, become dashed with surf, and then give way entirely to the sea, which, if
the day be at all fresh, will give the visitor a sprinkling.
Six hundred yards from the shore the inner breakwater ends in a noble
bastion-like head, rising, with smooth round sides, some thirty feet above the
waves. A space of four hundred feet
separates this head from its partner, the precisely similar work at the end of
the outer breakwater. The staging at this point is carried out a little to the
right (not passing over the heads, but swerving slightly from them) and is
narrowed to three lines of road instead of five; but, upon reaching the outer
limb of the work, the five lines immediately re-assemble, and go on together all
the rest of the way. This
intervening piece of three-line staging is the perfect part of the whole cage.
Its firm unyielding timbers will bear, almost without vibration, the
forty-eight tons of the four loaded wagons, and the weight of the engine, too.
The case is far different as they pass over the older timbers near the
shore, which are also unsupported by the iron rods found further on, and over
which the trains dance up and down as they pass, and seem to hover about the
extremest limit of safe passage.
From
the point where the five lines re-assemble, the whole course is free from
interruption to the further end. It
is a scene of bustle. Here, we pass
a gang of men preparing timber for the shores and brackets that support the
road-pieces; there, we see a man running along the narrow footway of the workmen
– a single plank laid on each side of the rails – as much at ease as if a
false step would not tumble him thirty feet down into the sea, or, worse, upon
the rugged rubbly heap; which, now emerging from the waves, indicates what the
nature of this outer arm is hereafter to be.
The inner breakwater is already being cased with dressed stone; but the
outer portion is to be left – at least, according to the present estimate –
as a rough slope of rubble, which will keep the sea out quite as well.
Every two or three minutes comes rumbling behind us a train, with its
four loaded wagons, each wagon averaging twelve tons in weight.
An ordinary load consists of a large block in the center, some two or
three feet in diameter, around which are heaped fragments of smaller sizes, the
whole rising to a considerable height in the wagon.
It is a fine thing to watch the tipping of the rubble through the open
rafters of the cage. Every wagon
has a dropping-floor, slanting downwards from back to front, but with its
iron-work lighter and less massive in front than behind.
It is so contrived that a brakeman, with a few blows of his hammer,
knocks away the check, and sets the floor free to drop; the front drops at once,
because, owing to its greater depth, it is pressed by the greater weight of
stone; the whole mass tumbles with a confused uproar upon the rubble-heap below,
and then the heavy iron-work behind causes the floor at once to return to its
natural position, in which it is immediately re-fastened. A puff or two of the engine brings each wagon in succession
over the required spot; and, unless the large stone should become jammed, the
whole load is tipped, and the empty train is on its way back, in less than a
minute. The jamming, when it
happens, is an awkward business, and men are sometimes at work for hours with
picks and crowbars before some obstinate mass will slip between the iron sides.
Such accidents are almost always the result of careless packing on the
part of the convicts at the top of the inclines; the process being, indeed, one
that demands not a little art and skill. When
the rubble embankment was still below the surface, the effect of the tipping was
greatly heightened by the fine hollow roar of the great plunge into the water,
and by the column of spray that was dashed high into the air.
As
you come near the present limit of the works, poles may be seen stuck upright
and painted in plain black and white, which indicate the precise direction to be
taken by the remainder of the work; and the eye, following the line of sight,
will rest upon the hills on the coast of Weymouth Bay, just at the point where a
colossal figure of George the Third on horseback has been scooped out in the
chalk. Standing upon the pathway
over the last tier of piles, and looking down, we may observe a weather-beaten
old man in a boat. His boat is
moored to one of the piles, and his duty is to keep watch, and be ready for
action in the event of anything or anybody falling over.
There
he sits chewing his quid, with a force of patience never to be surpassed, and
with a stomach certainly beyond the comprehension of a landsman.
It is the chief joy of his life, and commonly his only business, to pay
back with interest any amount of "chaff" that may drop on him from overhead.
A small wooden shed at this point of the works, raised a few feet above
the staging, contains a dioptric lantern: that is, a lantern furnished with
prismatic circles of glass for about a foot above and below the light, to catch
the rays and force them out in a direct line seaward, with an intense glare.
When the breakwater is complete, there will be at the passage, between
the inner and outer limbs on each head, such a lantern.
As
we return along the cage, we stop to watch "the travellers" at work, where
masons are setting the coping-stones of casing for the inner breakwater. Two small-wheeled trucks, perhaps eight feet apart, stand on
a line of rails. On a parallel
line, sixty feet distant, there are two similar trucks. From all four trucks uprights rise to the height of twelve or
fourteen feet, and across these uprights a platform is laid.
There are four winches, one outside each upright, by which four men can
move the whole machine up or down the two widely parted lines of rails which may
have two or even more lines lying between them.
This extensive apparatus is required for the support of a crane, but not
a common crane. It has a crane that
has no great arm reaching up into the air, but consists of a series of compact,
well-adjusted wheels on a small stand, which can be run upon rails up and down
the sixty feet of platform. Some of
the travellers are made still more complete by pivots at the top of each
upright, which allow one end of the platform to be wheeled a given distance
along its own set of rails, without compelling any movement at the other end. This is a machine used for setting the stone of the
breakwater casing. The crane will
hold a block of several tons weight neatly hewn for the cornice which is
crowning the six courses of granite wall below, and grip it fast while the
workmen adjust and re-adjust, enabled by this means to set with all the nicety
that could be used in the adjustment of a stone weighing pounds instead of tons.
A spirit-level is invariably used; and it was also employed
five-and-twenty feet below the surface of the water, by the diving masons, who,
in Deane's diving dress, adjusted the foundations of the splendidly-built
heads. Some notion may be formed of
the work bestowed upon the heads, by the fact that, though four hundred feet
asunder, six inches is the utmost difference between their levels.
Three hundred pounds is the lowest cost of one of the large travellers.
To
know what the cage is like, we should observe the work of pushing out a new bay,
or tier, or row of piles, from the end of the staging.
The piles, which are made in the yard, are formed of double timbers, the
two beams being securely bolted and tree-nailed together.
The pieces are scarfed; that is, cut so as to overlap and be joined even
or flush, and the whole pile is in section fourteen inches by twenty-eight.
As soon as it is made, each pile is thrust into an air-tight cylinder,
and, the air both from the cylinder and the pores of the wood being extracted by
means of an exhaust-pump, creosote is introduced instead of air.
A considerable pressure is put on, until the wood has absorbed the right
number of pounds of creosote to the hundredweight.
Trussed booms of at least sixty feet in length (huge rafters with
perpendicular pieces fixed beneath), are now rigged out from the present
staging, one boom from the center of each road, making five in all.
Each boom projects thirty feet overboard, that being the distance at
which the next bay of piles is to be constructed.
They are kept from swaying out of the proper direction by long pieces of
timber, some six inches square, fixed to their outer end and to a point on the
present staging.
The
booms being thus provided for, the piles are next towed out, with cast-iron
weights attached to the ends, in addition to the shoe and the Mitchell's
screw, with which they are to be screwed eight feet into the ground.
The ends, in consequence, sink; and the heads are hoisted up into the
jaw, or forked opening formed in the outer ends of the booms.
Thus the piles are held in position over the spot of ground to which they
must be screwed. Capstan heads are
on the heads of the piles, into which capstan bars are now put, having on the
end of each a small jaw or bird's-mouth, to bite the rope when inserted.
Wheeled-platforms, called trollies, are then run up to the head of the
staging, and fixed there. Each
trolley has a crab mounted, and firmly bolted upon it; that is, a set of winding
machinery, with a barrel, and winch, and spur-gear, increasing the power and
communicating motion from the winch to the barrel. Men are stationed at the crab, and as soon as they commence
winding, motion is given to the capstan-bars, and by them to the pile, which is
thus firmly screwed into the ground. Crossheads,
of double timbers like the piles, are now fitted into their upper ends, which
are formed so as to receive them, and the whole is securely bolted through.
Long cranes of thirty-feet gauge are used to drop these crossheads into
place. Tie-rods are also put
through the piles just above the level of low water mark, to give them a greater
degree of firmness. Trussed
road-pieces made in the yard can now be fitted athwart the cross roads, one on
either side of each pile; other timbers, called transoms and chocks, for
securing the road-way in its true position, are fitted in, and the narrow plank
for the workman's footway is attached to either side, and supported by
brackets. The cost of making and
fitting every single pile is about seventy pounds; and not less than twelve or
thirteen hundred constitute the staging as it now exists.
The general width of the breakwater staging where five roadways run is
one hundred and fifty feet; and the length of the piles at the outer end ninety
feet exclusive of shoe and screw, thus allowing, in ten fathom water, thirty
feet clear above the level of low water of ordinary, spring tides.
We have seen that the staging between the two heads, where three roads
only run, is steadier and less yielding to the weight of the wagons than that on
either side of it, but especially near the shore.
This arises from the outside pile only being trussed and stayed in the
bay or row of five piles, whereas in the rows of three all the piles are
supported thus; each pile is further strengthened by screw moorings, that is, by
long rods of iron reaching from the head of the timbers, and screwed into the
ground at a considerable angle.
There
is no room here for saying anything about the workshops, one great feature of
which is the circular saw that will cut through a forty feet plank in six
minutes, and we must only hint at the screw-breaks at the drum-heads, by which
the downward speed of loaded wagons is completely governed.
We give one good word to the beautiful weighing machine at the head of
the inclines, and the coaling jetty at their foot, the granite courses for which
are being now laid; and end the discourse with some authentic details as to the
state and extent of the work in general. The act for the construction of this bread-water was passed
in eighteen hundred and forty seven, and the first stone was deposited in the
summer of eighteen hundred and forty nine.
Since that period three millions of tons of stone have been deposited;
they can now tip nearly half a million per annum.
To quarry this rubble stone there are nine hundred and twenty-three, out
of the fifteen hundred convicts constantly at work.
The convicts never leave the summit of the hill.
The total number of men employed is three hundred and ninety-six; there
are also thirty or forty horses used about the works.
The description of work undertaken is, according to the act of parliament this; the formation of an inner and outer breakwater, together two thousand five hundred yards, (one mile seven hundred and forty yards,) in length, which will completely shelter two thousand one hundred and seven acres of Portland Bay; the depth over the anchorage varying from two to ten fathoms. The Admiralty decided that the entrance should be made available for men-of-war, and the largest steamers, and the heads are therefore founded at a clear depth of twenty-four feet at least, below low water of spring-tides, with the rubble slope down to the ground-surfaces beginning at that depth, so as to avoid the possibility of large vessels striking in a heavy sea. Of this it is evident there can be no risk whatever, since the lowest depth of water is about forty-five feet at the lowest tides. The total estimate of cost is below a million; Plymouth breakwater, the length of which is between five hundred and six hundred yards less than that of Portland, cost three-quarters of a million beyond the Portland estimate. The difference is due, of course mainly, to the astonishing advantages of Portland over almost every other site, but partly also to the advance of practical science since eighteen hundred and twelve. The date of the completion of this great national undertaking must manifestly depend, and is reported quarterly to the House of Commons as depending, on the degree to which use is made of convict labour in procuring stone.
Bob
Stone June 2003