s and thus greatly expedite the work of
rescue.

The telephone instruments in the helmet consist of one or more
loud-sounding receivers placed either in the crown of the helmet, or
one on each side in close proximity to the diver's ears. A transmitter
of a special watertight pattern is placed between the front glass and
one of the side glasses, and a contact piece, which, when the diver
presses his chin against it, rings a bell at the surface, is fitted
immediately below the front glass. A buzzer is sometimes fixed in the
helmet to call the diver's attention when the attendant wishes to
speak, but as a rule the voice is transmitted so loudly that this
device is unnecessary. A connexion, through which the insulated wires
connecting the instruments pass, terminates in contact pieces, and the
telephone cable, embedded in the diver's signal line, is connected
with it. The other end of the signal line is connected to a battery
box at the surface. This box contains, besides the cells, a receiver
and transmitter for the attendant, an electric bell, a terminal box,
and a special switch, by means of which various communications between
diver, or divers, and attendant are made. If, as is sometimes the
case, the diver happens to be somewhat deaf, he can, whilst he is
taking a message, stop the vibration of the outlet valve and the noise
made by the escaping air, by merely pressing his finger on a spindle
which passes through the disk of the valve, and thus momentarily
ensure absolute silence.

_Speaking Tube._--The rubber speaking tube which was the forerunner of
the telephonic apparatus is now practically obsolete, though it is
still used in isolated cases.

_Submarine Electric Lamps._--Various forms of submarine lamps are
used, from a powerful arc light to a self-contained hand lamp, the
former giving about 2000 or 3000 candle-power, and requiring a
steam-driven dynamo to supply the necessary current, the latter (fig.
5) giving a light of about 10 candle-power and having its own
batteries, so that the diver carries both the light and its source in
his hand. These submarine lamps are all constructed on the same
principle, having the incandescent lamps, or carbons as the case may
be, enclosed in a strong glass globe, the mechanism and connexions
being fitted in a metal case above the globe, which is flanged and
secured watertightly to the case.

_Self