Spanish mail steamer "Alphonso XII," sunk in
162 ft. of water off Las Palmas, Grand Canary; W. Ridyard recovered
L50,000 in silver dollars from the "Hamilton Mitchell," sunk off
Leuconna Reef, China, in 150 ft. There are individual cases where much
larger sums have been recovered, but those mentioned are particularly
notable by reason of the great depth involved and stand out as the
greatest depths at which good work has been done. The sponge fishers of
the Mediterranean work at a maximum depth of about 150 ft., and the
pearl divers of Australia at 120 ft. But submarine operations on the
great majority of the harbour and dock works of the world are conducted
at a depth of from 30 to 60 ft.

The weighted tools employed by divers differ very little from those used
by the workmen on _terra firma_. Pneumatic tools, worked by compressed
air conveyed from the surface through flexible tubes, are great aids,
particularly in rock removal work. With the rock drill the diver bores a
number of holes to a given depth, inserts in these the charges of
dynamite or other explosive used, attaches one end of a wire to a
detonator which is inserted in the charge, and then comes to the
surface. The boat from which he works is then moved away from the scene
of operations, paying out the wire attached to the detonators, and when
at a safe distance the free end of the wire is connected to a magneto
exploding machine, which is then set in motion.

A complete set of diving apparatus costs from L75 to L200, varying with
the depth of water for which it is required.

The pay of a diver depends upon the nature of the work upon which he is
engaged, and also upon the depth of the water. On harbour and dock work
the average wage is 2s. to 2s. 6d. per hour; on wreck work from 3s. to
5s. an hour, according to depth; on treasure and cargo recovery so much
per day, with a percentage on the value recovered, generally about 5%.
The pearl fishers of Australia get so much per ton of shell, and the
sponge fishers are also paid by results.

A problem which has been exercising the minds of those engaged in
submarine work is the greatest depth at which it is possible to work,
for, as is well known, many a fine vessel with valuable cargo and
treasure is lying out of reach of the diver owing to the pressure which
he would have to sustain were he to attempt to reach her. Mr Leonard
Hill, and Drs Greenwood and J. J. R. Macleod conducted experiments in
conjunction