o 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 posit