The diver must be careful in his movements
that he does not fall so as suddenly to increase the depth of
water in which he is immersed, because at the normal higher
level the air pressure in the dress will be properly balanced
against the water pressure; but if he falls, say 30 ft, the
pressure of the water on his body will be increased by about 15
lb per square inch, and as the air pump cannot immediately
increase the pressure in the dress to a corresponding extent,
the man's body in the unresisting dress will be forced into the
rigid helmet, and he will certainly be severely injured, and
perhaps even killed.
When descending under water the air pressure in the dress is
increased, and acts upon the outside of the drum of the ear,
causing pain, until the air passing through the nose and up the
Eustachian tube inside the head reaches the back of the drum
and balances the pressure. This may be delayed, or prevented,
if the tube is partially stopped up by reason of a cold or
other cause, but the balance can generally be brought about if
the diver pauses in his descent and swallows his saliva; or
blocks up his nose as much as possible by pressing it against
the front of the helmet, closing the mouth and then making a
strong effort at expiration so as to produce temporarily an
extra pressure inside the throat, and so blow open the tubes;
or by yawning or going through the motions thereof.
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