greater the depth the greater the quantity of air required by the
diver. The pattern most generally in favour amongst divers of all
classes is a three-cylinder single-acting pump, which is suitable for
almost every description of work which the diver may be called upon to
perform, either in deep or shallow water. Another most useful type is
a two-cylinder double-acting pump (figs. 1 and 2), which is designed
to supply two divers working simultaneously in moderate depths of
water, or one diver only in deep water. An air-distributing
arrangement is fitted, whereby, when it is desired to send two men
down together, each cylinder supplies air independently of the other;
and when it is required to send one diver into deep water, the two
cylinders are connected and the full volume of air from both is
delivered to the one man. The same duty is also performed by a
four-cylinder single-acting pump. Smaller pumps, having one
double-acting or two single-acting cylinders, are also used for
shallow water work.

In most cases these air pumps are worked by manual power; this method
of working is rendered necessary by the fact that the machines are
usually placed in small boats from which the divers work and on which
other motive power is not available. In cases, however, where steam or
electric power is available the pumps are sometimes worked by their
means--more particularly on harbour and dock works. In such instances
the air is not delivered direct from the pump to the diver, but is
delivered into an intermediate steel receiver to which the diver's air
pipe is connected, the object being to ensure a reserve supply of air
in case of a breakdown of the pump. Some of these combinations of
pumps and motors are so arranged that, in the event of an accident to
the motor, the pump can be thrown out of gear with it, and be
immediately worked by hand power. Each pump is fitted with a gauge (or
gauges), indicating not only the pressure of air which the pump is
supplying, but also the depth of water at which the diver is working.
The cylinders are water-jacketed to ensure the air delivered to the
diver being cool, the water being drawn in and circulated round the
cylinders by means of a small metal pump worked from an eccentric on
the main crank-shaft. Filters are sometimes attached to the suction
and delivery sides of the pumps to ensure the inlet of air being free
f