Book: Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4

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CAISSON (from the Fr. _caisse_, the variant form "cassoon" being adapted
from the Ital. _casone_), a chest or case. When employed as a military
term, it denotes an ammunition wagon or chest; in architecture it is the
term used for a sunk panel or coffer in a ceiling, or in the soffit of an
arch or a vault.

In civil engineering, however, the word has attained a far wider
signification, and has been adopted in connexion with a considerable
variety of hydraulic works. A caisson in this sense implies a case or
enclosure of wood or iron, generally employed for keeping out water during
the execution of foundations and other works in water-bearing strata, at
the side of or under rivers, and also [v.04 p.0958] in the sea. There are
two distinct forms of this type of caisson:--(1) A caisson open at the top,
whose sides, when it is sunk in position, emerge above the water-level, and
which is either provided with a water-tight bottom or is carried down, by
being weighted at the top and having a cutting edge round the bottom, into
a water-tight stratum, aided frequently by excavation inside; (2) A
bottomless caisson, serving as a sort of diving-bell, in which men can work
when compressed air is introduced to keep out the water in proportion to
the depth below the water-level, which is gradually carried down to an
adequately firm foundation by excavating at the bottom of the caisson, and
building up a quay-wall or pier out of water on the top of its roof as it
descends. An example of a caisson with a water-tight bottom is furnished by
the quays erected alongside the Seine at Rouen, where open-timber caissons
were sunk on to bearing-piles down to a depth of 93/4 ft. below low-water,
the brick and concrete lower portions of the quay-wall being built inside
them out of water (see DOCK). At Bilbao, Zeebrugge and Scheveningen
harbours, large open metal caissons, built inland, ballasted with concrete,
floated out into position, and then sunk and filled with concrete, have
been employed for forming very large foundation blocks for the breakwaters
(see BREAKWATER). Open iron caissons are frequently employed for enclosing
the site of river piers for bridges, where a water-tight stratum can be
reached at a moderate depth, into which the caisson can be taken down, so
that the water can be pumped out of the enclosure and the foundations laid
and the pier carried up in the open air. Thus the two large river piers
carrying the high towers, bascules, and machinery of the Tower Bridge,
London, were each founded and built within a group of twelve plate-iron
caissons open at the top; whilst four of the piers on which the cantilevers
of the Forth Bridge rest, were each erected within an open plate-iron
caisson fitted at the bottom to the sloping rock, where ordinary cofferdams
could not have been adopted.

Where foundations have to be carried down to a considerable depth in
water-bearing strata, or through the alluvial bed of a river, to reach a
hard stratum, bottomless caissons sunk by excavating under compressed air
are employed. The caisson at the bottom, forming the working chamber, is
usually provided with a strong roof, round the top of which, when the
caisson is floated into a river, plate-iron sides are erected forming an
upper open caisson, inside which the pier or quay-wall is built up out of
water, on the top of the roof, as the sinking proceeds. Shafts through the
roof up to the open air provide access for men and materials to the working
chamber, through an air-lock consisting of a small chamber with an
air-tight door at each end, enabling locking into and out of the
compressed-air portion to be readily effected, on the same principle as a
water-lock on a canal. When a sufficiently reliable stratum has been
reached, the men leave the working chamber; and it is filled with concrete
through the shafts, the bottomless caisson remaining embedded in the work.
The foundations for the two river piers of the Brooklyn Suspension Bridge,
carried down to the solid rock, 78 and 45 ft. respectively below
high-water, by means of bottomless timber caissons with compressed air,
were an early instance of this method of carrying out subaqueous
foundations; whilst the Antwerp quay-walls, commenced many years ago in the
river Scheldt at some distance out from the right bank, and the foundations
of six of the piers supporting the cantilevers of the Forth Bridge, carried
down to rock between 64 and 89 ft. below high-water, are notable examples
of works founded under water within wrought iron bottomless caissons by the
aid of compressed air. The foundations of the two piers of the Eiffel Tower
adjoining the Seine were carried down through soft water-bearing strata to
a depth of 33 ft. by means of wrought iron bottomless caissons sunk by the
help of compressed air; and the deep foundations under the sills of the new
large Florida lock at Havre (see DOCK) were laid underneath the water
logged alluvial strata close to the Seine estuary by similar means.
Workmen, after emerging from such caissons, sometimes exhibit symptoms of
illness which is known as _caisson disease_ (_q.v._).

As in the above system, significantly termed by French engineers _par
caisson perdu_, the materials of the bottomless caisson have to be left in
the work, a more economical system has been adapted for carrying out
similar foundations, at moderate depths, by using movable caissons, which,
after the lowest portions of the foundations have been laid, are raised by
screw-jacks for constructing the next portions. In this way, instead of
building the pier or wall on the roof of the caisson, the work is carried
out under water in successive stages, by raising the bottomless caisson as
the work proceeds; and by this arrangement, the caisson, having completed
the subaqueous portion of the structure, is available for work elsewhere.
This movable system has been used with advantage for the foundations for
some piers of river bridges, some breakwater foundations, and, at the
Florida lock, Havre, for founding portions of the side walls.

Closed iron caissons, termed ship-caissons, and sliding or rolling
caissons, are generally employed for closing graving-docks, especially the
former (so called from their resemblance in shape to a vessel) on account
of their simplicity, being readily floated into and out of position; whilst
sliding caissons are sometimes used instead of lock-gates at docks, but
require a chamber at the side to receive them when drawn back. They possess
the advantage, particularly for naval dockyards where heavy weights are
transported, of providing in addition a strong movable bridge, thereby
dispensing with a swing-bridge across the opening.

The term caisson is sometimes applied to flat air-tight constructions used
for raising vessels out of water for cleaning or repairs, by being sunk
under them and then floated; but these floating caissons are more commonly
known as pontoons, or, when air-chambers are added at the sides, as
floating dry-docks.

(L. F. V.-H.)

CAISSON DISEASE. In order to exclude the water, the air pressure within a
caisson used for subaqueous works must be kept in excess of the pressure
due to the superincumbent water; that is, it must be increased by one
atmosphere, or 15 lb per sq. in. for every 331/2 ft. that the caisson is
submerged below the surface. Hence at a depth of 100 ft. a worker in a
caisson, or a diver in a diving-dress, must be subjected to a pressure of
four atmospheres or 60 lb per sq. in. Exposure to such pressures is apt to
be followed by disagreeable and even dangerous physiological effects, which
are commonly referred to as caisson disease or compressed air illness. The
symptoms are of a very varied character, including pains in the muscles and
joints (the "bends"), deafness, embarrassed breathing, vomiting, paralysis
("divers' palsy"), fainting and sometimes even sudden death. At the St
Louis bridge, where a pressure was employed equal to 41/4 atmospheres, out of
600 workmen, 119 were affected and 14 died. At one time the symptoms were
attributed to congestion produced by the mechanical effects of the pressure
on the internal organs of the body, but this explanation is seen to be
untenable when it is remembered that the pressure is immediately
transmitted by the fluids of the body equally to all parts. They do not
appear during the time that the pressure is being raised nor so long as it
is continued, but only after it has been removed; and the view now
generally accepted is that they are due to the rapid effervescence of the
gases which are absorbed in the body-fluids during exposure to pressure.
Experiment has proved that in animals exposed to compressed air nitrogen is
dissolved in the fluids in accordance with Dalton's law, to the extent of
roughly 1% for each atmosphere of pressure, and also that when the pressure
is suddenly relieved the gas is liberated in bubbles within the body. It is
these bubbles that do the mischief. Set free in the spinal cord, for
instance, they may give rise to partial paralysis, in the labyrinth of the
ear to auditory vertigo, or in the heart to stoppage of the circulation; on
the other hand, they may be liberated in positions where they do no harm.
But if the pressure is relieved gradually they are not formed, because the
gas comes out of solution slowly and is got rid of by the heart and lungs.
Paul Bert exposed 24 dogs to pressure of 7-91/2 atmospheres and
"decompressed" them rapidly in 1-4 minutes. The result was that 21 died,
while only one showed no symptoms. In one of his cases, in which the
apparatus burst while at a pressure of 91/2 atmospheres, death was
instantaneous and the body was enormously distended, with the right heart
full of gas. [v.04 p.0959] But he also found that dogs exposed, for
moderate periods, to similar pressures suffered no ill effects provided
that the pressure was relieved gradually, in 1-11/2 hours; and his results
have been confirmed by subsequent investigators. To prevent caisson
disease, therefore, the decompression should be slow; Leonard Hill suggests
it should be at a rate of not less than 20 minutes for each atmosphere of
pressure. Good ventilation of the caisson is also of great importance
(though experiment does not entirely confirm the view that the presence of
carbonic acid to an amount exceeding 1 or 11/4 parts per thousand exercises a
specific influence on the production of compressed air illness), and long
shifts should be avoided, because by fatigue the circulatory and
respiratory organs are rendered less able to eliminate the absorbed gas.
Another reason against long shifts, especially at high pressures, is that a
high partial pressure of oxygen acts as a general protoplasmic poison. This
circumstance also sets a limit to the pressures that can possibly be used
in caissons and therefore to the depths at which they can be worked, though
there is reason to think that the maximum pressure (43/4 atmospheres) so far
used in caisson work might be considerably exceeded with safety, provided
that proper precautions were observed in regard to slow decompression, the
physique of the workmen, and the hours of labour. As to the remedy for the
symptoms after they have appeared, satisfactory results have been obtained
by replacing the sufferers in a compressed air chamber ("recompression"),
when the gas is again dissolved by the body fluids, and then slowly
"decompressing" them.

See Paul Bert, _La Pression barometrique_ (1878); and Leonard Hill, _Recent
Advances in Physiology and Biochemistry_ (1906), (both these works contain
bibliographies); also a lecture by Leonard Hill delivered at the Royal
Institution of Great Britain on the 25th of May 1906; "Diving and Caisson
Disease," a summary of recent investigations, by Surgeon Howard Mummery,
_British Medical Journal_, June 27th, 1908; _Diseases of Occupation_, by T.
Oliver (1908); _Diseases of Workmen_, by T. Luson and R. Hyde (1908).

CAITHNESS, a county occupying the extreme north-east of Scotland, bounded
W. and S. by Sutherlandshire, E. by the North Sea, and N. by the Pentland
Firth. Its area is 446,017 acres, or nearly 697 sq. m. The surface
generally is flat and tame, consisting for the most part of barren moors,
almost destitute of trees. It presents a gradual slope from the north and
east up to the heights in the south and west, where the chief mountains are
Morven (2313 ft.), Scaraben (2054 ft.) and Maiden Pap (1587 ft.). The
principal rivers are the Thurso ("Thor's River"), which, rising in Cnoc
Crom Uillt (1199 ft.) near the Sutherlandshire border, pursues a winding
course till it reaches the sea in Thurso Bay; the Forss, which, emerging
from Loch Shurrery, follows a generally northward direction and enters the
sea at Crosskirk, a fine cascade about a mile from its mouth giving the
river its name (_fors_, Scandinavian, "waterfall;" in English the form is
_force_); and Wick Water, which, draining Loch Watten, flows into the sea
at Wick. There are many other smaller streams well stocked with fish.
Indeed, the county offers fine sport for rod and gun. The lochs are
numerous, the largest being Loch Watten, 23/4 m. by 3/4 m., and Loch Calder, 21/4
by 1 m., and Lochs Colam, Hempriggs, Heilen, Ruard, Scarmclate, St John's,
Toftingale and Wester. So much of the land is low-lying and boggy that
there are no glens, except in the mountainous south-west, although towards
the centre of the county are Strathmore and Strathbeg (the great and little
valleys). Most of the coast-line is precipitous and inhospitable,
particularly at the headlands of the Ord, Noss, Skirsa, Duncansbay, St
John's Point, Dunnet Head (346 ft.), the most northerly point of Scotland,
Holburn and Brims Ness. From Berriedale at frequent intervals round the
coast occur superb "stacks," or detached pillars of red sandstone, which
add much to the grandeur of the cliff scenery.

Caithness is separated from the Orkneys by the Pentland Firth, a strait
about 14 miles long and from 6 to 8 miles broad. Owing to the rush of the
tide, navigation is difficult, and, in rough weather, dangerous. The tidal
wave races at a speed which varies from 6 to 12 m. an hour. At the meeting
of the western and eastern currents the waves at times rise into the air
like a waterspout, but the current does not always nor everywhere flow at a
uniform rate, being broken up at places into eddies as perilous as itself.
The breakers caused by the sunken reefs off Duncansbay Head create the
Bores of Duncansbay, and eddies off St John's Point are the origin of the
Merry Men of Mey, while off the island of Stroma occurs the whirlpool of
the Swalchie, and off the Orcadian Swona is the vortex of the Wells of
Swona. Nevertheless, as the most direct road from Scandinavian ports to the
Atlantic the Firth is used by at least 5000 vessels every year. In the
eastern entrance to the Firth lies the group of islands known as the
Pentland Skerries. They are four in number--Muckle Skerry, Little Skerry,
Clettack Skerry and Louther Skerry--and the nearest is 41/2 m. from the
mainland. On Muckle Skerry, the largest (1/2 m. by 1/3 m.), stands a
lighthouse with twin towers, 100 ft. apart. The island of Stroma, 11/2 m.
from the mainland (pop. 375), belongs to Caithness and is situated in the
parish of Canisbay. It is 21/4 m. long by 11/4 m. broad. In 1862 a remarkable
tide climbed the cliffs (200 ft.) and swept across the island.

_Geology._--Along the western margin of the county from Reay on the north
coast to the Scaraben Hills there is a narrow belt of country which is
occupied by metamorphic rocks of the types found in the east of Sutherland.
They consist chiefly of granulitic quartzose schists and felspathic
gneisses, permeated in places by strings and veins of pegmatite. On the
Scaraben Hills there is a prominent development of quartz-schists the age
of which is still uncertain. These rocks are traversed by a mass of granite
sometimes foliated, trending north and south, which is traceable from Reay
southwards by Aultnabreac station to Kinbrace and Strath Helmsdale in
Sutherland. Excellent sections of this rock, showing segregation veins, are
exposed in the railway cuttings between Aultnabreac and Forsinard. A rock
of special interest described by Professor Judd occurs on Achvarasdale
Moor, near Loch Scye, and hence named Scyelite. It forms a small isolated
boss, its relations to the surrounding rocks not being apparent. Under the
microscope, the rock consists of biotite, hornblende, serpentinous
pseudo-morphs after olivine and possibly after enstatite and magnetite, and
may be described as a mica-hornblende-picrite. The remainder of the county
is occupied by strata of Old Red Sandstone age, the greater portion being
grouped with the Middle or Orcadian division of that system, and a small
area on the promontory of Dunnet Head being provisionally placed in the
upper division. By means of the fossil fishes, Dr Traquair has arranged the
Caithness flagstone series in three groups, the Achanarras beds at the
base, the Thurso flagstones in the middle, and the John o' Groats beds at
the top. In the extreme south of the county certain minor subdivisions
appear which probably underlie the lowest fossiliferous beds containing the
Achanarras fauna. These comprise (1) the coarse basement conglomerate, (2)
dull chocolate-red sandstones, shales and clays around Braemore in the
Berriedale Water, (3) the brecciated conglomerate largely composed of
granite detritus seen at Badbea, (4) red sandstones, shales and
conglomeratic bands found in the Berriedale Water and further northwards in
the direction of Strathmore. Morven, the highest hill in Caithness, is
formed of gently inclined sandstones and conglomerates resting on an eroded
platform of quartz-schists and quartz-mica-granulites. The flagstones
yielding the fishes of the lowest division of the Orcadian series appear on
Achanarras Hill about three miles south of Halkirk. The members of the
overlying Thurso group have a wide distribution as they extend along the
shore on either side of Thurso and spread across the county by Castletown
and Halkirk to Sinclairs Bay and Wick. They are thrown into folds which are
traversed by faults some of which run in a north and south direction. They
consist of dark grey and cream-coloured flagstones, sometimes thick-bedded
with grey and blue shales and thin limestones and occasional intercalations
of sandstone. In the north-west of the county the members of the Thurso
group appear to overlap the Achanarras beds and to rest directly on the
platform of crystalline schists. In the extreme north-east there is a
passage upwards into the John o' Groats group [v.04 p.0960] with its
characteristic fishes, the strata consisting of sandstones, flagstones with
thin impure limestones. The rocks of Dunnet Head, which are provisionally
classed with the upper Old Red Sandstone, are composed of red and yellow
sandstones, marls and mudstones. Hitherto no fossils have been obtained
from these beds save some obscure plant-like markings, but they are
evidently a continuation southwards of the sandstones of Hoy, which there
rest unconformably on the flagstone series of Orkney. This patch of Upper
Old Red strata is faulted against the Caithness flagstones to the south.
For many years the flagstones have been extensively quarried for pavement
purposes, as for instance near Thurso, at Castletown and Achanarras. Two
instances of volcanic necks occur in Caithness, one piercing the red
sandstones at the Ness of Duncansbay and the other the sandstones of Dunnet
Head north of Brough. They point to volcanic activity subsequent to the
deposition of the John o' Groats beds and of the Dunnet sandstones. The
materials filling these vents consist of agglomerate charged with blocks of
diabase, sandstone, flagstone and limestone.

An interesting feature connected with the geology of Caithness is the
deposit of shelly boulder clay which is distributed over the low ground,
being deepest in the valleys and in the cliffs surrounding the bays on the
east coast. Apart from the shell fragments, many of which are striated, the
deposit contains blocks foreign to the county, as for instance chalk and
chalk-flints, fragments of Jurassic rocks with fossils and pieces of jet.
The transport of local boulders shows that the ice must have moved from the
south-east towards the north-west, which coincides with the direction
indicated by the striae. The Jurassic blocks may have been derived from the
strip of rocks of that age on the east coast of Sutherland. The shell
fragments, many of which are striated, include arctic, boreal and southern
forms, only a small number being characteristic of the littoral zone.

_Climate and Agriculture._--The climate is variable, and though the winter
storms fall with great severity on the coast, yet owing to proximity to a
vast expanse of sea the cold is not intense and snow seldom lies many days
continuously. In winter and spring the northern shore is subject to
frequent and disastrous gales from the N. and N.W. Only about two-fifths of
the arable land is good. In spite of this and the cold, wet and windy
climate, progressive landlords and tenants keep a considerable part of the
acreage of large farms successfully tilled. In 1824 James Traill of Ratter,
near Dunnet, recognizing that it was impossible to expect tenants to
reclaim and improve the land on a system of short leases, advocated large
holdings on long terms, so that farmers might enjoy a substantial return on
their capital and labour. Thanks to this policy and the farmers' skill and
enterprise, the county has acquired a remarkable reputation for its
produce; notably oats and barley, turnips, potatoes and beans.
Sheep--chiefly Leicester and Cheviots--of which the wool is in especial
request in consequence of its fine quality, cattle, horses and pigs are
raised for southern markets.

_Other Industries._--The great source of profit to the inhabitants is to be
found in the fisheries of cod, ling, lobster and herring. The last is the
most important, beginning about the end of July and lasting for six weeks,
the centre of operations being at Wick. Besides those more immediately
engaged in manning the boats, the fisheries give employment to a large
number of coopers, curers, packers and helpers. The salmon fisheries on the
coast and at the mouths of rivers are let at high prices. The Thurso is one
of the best salmon streams in the north. The flagstone quarries, mostly
situated in the Thurso, Olrig and Halkirk districts, are another important
source of revenue. Of manufactures there is little beyond tweeds, ropes,
agricultural implements and whisky, and the principal imports consist of
coal, wood, manure, flour and lime.

The only railway in the county is the Highland railway, which, from a point
some four miles to the south-west of Aultnabreac station, crosses the shire
in a rough semicircle, via Halkirk, to Wick, with a branch from Georgemas
Junction to Thurso. There is also, however, frequent communication by
steamer between Wick and Thurso and the Orkneys and Shetlands, Aberdeen,
Leith and other ports. The deficiency of railway accommodation is partly
made good by coach services between different places.

_Population and Government._--The population of Caithness in 1891 was
33,177, and in 1901, 33,870, of whom twenty-four persons spoke Gaelic only,
and 2876 Gaelic and English. The chief towns are Wick (pop. in 1901, 7911)
and Thurso (3723). The county returns one member to parliament. Wick is the
only royal burgh and one of the northern group of parliamentary burghs
which includes Cromarty, Dingwall, Dornoch, Kirkwall and Tain. Caithness
unites with Orkney and Shetland to form a sheriffdom, and there is a
resident sheriff-substitute at Wick, who sits also at Thurso and Lybster.
The county is under school-board jurisdiction, and there are academies at
Wick and Thurso. The county council subsidizes elementary schools and
cookery classes and provides apparatus for technical classes.

_History._--The early history of Caithness may, to some extent, be traced
in the character of its remains and its local nomenclature. Picts' houses,
still fairly numerous, Norwegian names and Danish mounds attest that these
peoples displaced each other in turn, and the number and strength of the
fortified keeps show that its annals include the usual feuds, assaults and
reprisals. Circles of standing stones, as at Stemster Loch and Bower, and
the ruins of Roman Catholic chapels and places of pilgrimage in almost
every district, illustrate the changes which have come over its
ecclesiastical condition. The most important remains are those of Bucholie
Castle, Girnigo Castle, and the tower of Keiss; and, on the S.E. coast, the
castles of Clyth, Swiney, Forse, Laveron, Knockinnon, Berriedale, Achastle
and Dunbeath, the last of which is romantically situated on a detached
stack of sandstone rock. About six miles from Thurso stand the ruins of
Braal Castle, the residence of the ancient bishops of Caithness. On the
coast of the Pentland Firth, 11/2 miles west of Dunscansbay Head, is the site
of John o' Groat's house.

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