rom dirt, and the discharge of air free from dirt and oil.

[Illustration: FIG. 2.--Pump in chest, ready for work.]

_Helmet._--The helmet and breastplate (fig. 3) are made from highly
planished tinned copper, with gun-metal valves and other fittings. The
helmet is provided with a non-return air inlet valve to which the
diver's air pipe is connected; the air when it lifts the inlet valve
passes through three conduits--one having its outlet over the front
glass, the others their outlets over the side glasses. In this way the
diver gets the air fresh as it enters the helmet, and at the same time
it prevents condensation of his breath on the glasses and keeps them
clear. There is a regulating air outlet valve by which the diver
adjusts his supply of air according to his requirements in different
depths of water; the valve is usually made to be adjusted by hand, but
sometimes it is so constructed as to be operated by the diver knocking
his head against it, the spindle being extended through to the inside
of the helmet and fitted at its inner extremity with a button or disk.
By unscrewing the valve, the diver allows air to escape, and thus the
dress is deflated; by screwing it up the air is retained and the dress
inflated. Thus the diver can control his specific gravity and rise or
sink at will. In case by any chance the diver should inflate the dress
inadvertently, and wish to get rid of the superfluous air quickly, he
can do so by opening an emergency cock, which is fitted on the helmet.
Plate glasses in gun-metal frames are also fitted to the helmet, two,
one on each side, being permanently fixed, while one in front is made
either to screw in and out, or to work on a hinged joint like a ship's
scuttle; the side glasses are usually protected by metal cross-bars,
as is also sometimes the front glass. Some divers prefer unprotected
glasses at the side of the helmet, instead of protected oval ones.

The breastplate is fitted on its outer edge with metal screws and
bands. The disposition of the screws corresponds with that of the
holes in the india-rubber collar of the diving dress described below.
There are other methods of making a watertight joint between the
diver's breastplate and the diving dress, but, as these are only
mechanical differences, it will suffice to describe the Siebe-Gorman
apparatus, as exclusively adopted by the British government. Whatev