366	NICHOLLS'S  SEAMANSHIP  AND   NAUTICAL  KNOWLEDGE
adjusted by means of an air pump to withstand the collapsing pressure
of 64 Ibs. per square foot for every foot depth of water he descends.
Water is Non-Compressible.—Air is compressible, but water under
normal conditions is not so in practice. The average pressure of the
atmosphere is 15 Ibs per square inch which works out afc 2160 Ibs pei
square foot, or nearly 1 ton, so that an average sized person having a
surface body area of 10 square feet is built to withstand a normal
pressure of about 10 tons, but the air pressure inside his body is equal
to the external pressure and so he ib able to carry on.
The Atmospheric Sounding Tube is based on the compressibility of
the air entrapped in the glass tube, the basis being Boyle's Law, which
states "that the volume of a gas varies inversely as the pressure when
the temperature is constant."    This may be written
New pressure       old volume
Old pressure       new volume
The sounding tubes are 24 ins, long and their bores are exactly
parallel throughout their length, one end being sealed and the tube 13
lowered into the sea open end downwards. The air in the tube exerts
a pressure of 15 Ibs. per square inch, which is equivalent to the pressure
exerted by water at a depth of 33 feet. The pressure
at sea surface is 1 atmosphere     = 15 Ibs. per sq. in.
at 33 ft. down it is 2 atmospheres   = 30	„
at 66 ft. down it is 3 atmospheres = 45	„	etc.
When the tube descends the pressure of the water increases and
rises in the tube, thus gradually reducing the volume of the entrapped
air, the minimuni volume being recorded when the maximum pressure
is reached, namely, on reaching the bottom. The tube records the
volume by means of the discoloration; the volume gives the pressure
and the pressure gives the depth.
Example.—The volume of air left in a 24 in. tube is 6 inches after
-t-t-      reaching the bottom, required the depth of water.
Let x represent the depth corresponding to the increase
of pressure.
New pressure       old volume
^	Old pressure        new volume
2*	^   a+33 ft.     __ 24 ins.
33 ft.       ~~    6 ins.
x+33=33x 24^-6=132 ft.
x=132-33=99 ft.=16i fens,
L— I        Fig. 7.