The diving bell is a pneumatic engine, by means of which persons can
descend to great depths in the sea, and recover from it valuable
portions of wrecks and other things. Its principle may be well
illustrated by the following experiment. Take a glass tumbler, and
plunge it into the water with the mouth downwards, and it will be
found that the water will not rise much more than half way in the
tumbler. This may be made very evident if a piece of cork be suffered
to float inside the glass on the surface of the water. The air
within the tumbler does not entirely exclude the water, because air
is elastic, and consequently compressible, and hence the air in the
tumbler is what is called condensed. The diving bell is formed upon the
above principle; but instead of being glass it is a wooden or metal
vessel, of very large dimensions, so as to hold three or four persons,
who are supplied with air from above by means of powerful pumps, whilst
the excess of air escapes at the bottom of the bell.