alve, to which one end of a flexible tube was attached, the
other end being connected at the surface with a pump which supplied the
diver with a constant stream of fresh air. The air, which kept the water
well down, forced its way between the jacket and the under-garment, and
escaped to the surface on exactly the same principle as that of the
diving bell; hence the term "open" as applied to this dress.

Although most excellent work was accomplished with this dress--work
which could not be attempted before its introduction--it was still far
from perfect. It was absolutely necessary for the diver to maintain an
upright, or but very slightly stooping, position whilst under water; if
he stumbled and fell, the water filled his dress, and, unless quickly
brought to the surface, he was in danger of being drowned. To overcome
this and other defects, Siebe carried out a large number of experiments
extending over several years, which culminated, in the year 1830, in the
introduction of his "close" dress in combination with a helmet fitted
with air inlet and regulating outlet valves.

Though, of course, vast improvements have been introduced since Siebe's
death, in 1872, the fact remains that his principle is in universal use
to this day. The submarine work which it has been instrumental in
accomplishing is incalculable. But some idea of the importance of the
invention may be gathered from the fact that diving apparatus on Siebe's
principle is universally used to-day in harbour, dock, pier and
breakwater construction, in the pearl and sponge fisheries, in
recovering sunken ships, cargo and treasure, and that every ship in the
British navy and in most foreign navies carries one set or more of
diving apparatus.

A modern set of diving apparatus consists essentially of six parts:--(1)
an air pump, (2) a helmet with breastplate, (3) a diving dress, (4) a
pair of heavily weighted boots, (5) a pair of back and chest weights,
(6) a flexible non-collapsible air tube.

[Illustration: FIG. 1.--Pump out of chest.

Two-cylinder, Double-action Air Pump for Two Divers.

A, Air-distributing arrangement, for one diver or two divers.
B, Water jacket.
C, Suction and discharge valves.
D, Cylinders.
E, Pressure gauges.
F, Nozzles to which divers' air pipes are attached.]

_Air Pumps._--The type of air pump varies with the depth of water to
which the diver has to descend; it will be readily understood that the