ing to the depth at which the bell happened to
be working. The maximum depth on the Dover works was between 60 and 70
ft., = about 25-30 lb. to the square inch. A bell was lowered by
means of powerful steam-driven cranes, travelling on a gantry, to
within a few feet of the water, and the men entered it from a boat.
The bell then continued its descent to the bottom, where the men, with
pick and shovel, levelled the sea bed ready to receive the large
concrete blocks, weighing from 30 to 42 tons apiece. Having completed
one section, the bell was moved along to another. The concrete blocks
were then lowered and placed in position by helmet divers. The bell
divers, clad in thick woollen suits and watertight thigh boots, worked
in shifts of about three hours each, and were paid at the rate of from
1s. to 15d. per hour.

[Illustration: FIG. 7.--Air-lock Diving Bell.

A, Working chamber.
B, Air-lock.
C, Pulleys and wire ropes for lowering and raising bell.
D, Iron ladder.
E, Tackles suspended from roof for raising and lowering objects.
F, Air supply pipe.]

The cost of an ordinary diving bell, including air compressor,
telephonic apparatus and electric light, is from L600 to L1500,
according to size.

The _Air-lock Diving Bell_ (fig. 7) comprises an iron or steel working
chamber similar to the ordinary diving bell, but with the addition of
a shaft attached to its roof. At the upper end of the shaft is an
airtight door, and about 8 ft. below this is another similar door.
When the bell divers wish to enter the bell, they pass through the
first door and close it after them, and then open a cock or valve and
gradually let into the space between the two doors compressed air from
the working chamber in order to equalize the pressure; they then open
the second door and pass down into the working chamber, closing the
door after them. When returning to the surface they reverse the
operation. It can readily be imagined that, owing to its unwieldy
character, the employment of the air-lock bell is resorted to only in
those cases where the nature of the sea bed necessitates its remaining
on a given spot for some considerable time, as for instance in the
excavation of hard rock to a given depth.

An air-lock bell supplied to the British Admiralty, for use in
connexion with the laying of moorings at Gibraltar, has a working
chamber measuring 15