this particular capacity at the depth where they expected to dive; a
diver could work only fifteen minutes at 120 feet without requiring
decompression, and seventy cubic feet of air would last just long
enough. Double tanks would have meant the boys would be able to stay
down nearly twice as long, but would also have meant the nuisance of
waiting through the decompression period of about thirteen minutes ten
feet below the surface on the ascent. For this reason, the boys planned
to dive with single tanks, leaving the spares on the surface.

Of course, to get even fifteen minutes of diving at twenty fathoms the
tanks had to be filled to capacity. When full, they were under enormous
internal pressure of over two thousand pounds per square inch. The tanks
had been filled at Spindrift, but the boys decided to check them again,
in case there had been some leakage through the valves during shipment.

Scotty swung one tank upright and prepared to attach the pressure gauge.
Rick, inspecting another tank for bumps that might have weakened the
tank wall, saw him do it.

For a moment Rick continued his inspection, then what he had seen
suddenly registered. He yelled, "Scotty! The valve!"

In that instant, as Scotty attached the pressure gauge, the valve blew
out!

The entire valve assembly and the pressure gauge, propelled by the
tremendous pressure in the tank, blew straight upward, ripping clear of
Scotty's hand and taking a patch of skin along. The ascending assembly,
traveling with bullet speed, clipped a lock of hair from his bent head.

[Illustration: _The valve assembly, traveling with bullet speed, barely
missed Scotty's head_]

Scotty yelled, "Run!"

The tank, its air free to escape, writhed and turned, then fell over on
its side. It was like an inflated balloon, turned loose to fly around a
room. Air jetted from it with terrific velocity, so that the tank was,
for the period while its air lasted, a true rocket.

It struck the wall of the shed and went through it like paper, smashed
into a stud and caromed slightly, so that its trajectory was altered
enough to drive it directly at Rick. He fell flat and it went over, just
grazing him, then flew into the palm grove. It hit a palm a slanting
blow and turned upward, shooting high in the air, clipping off the top
of another palm as it went.

As the boys watched, horrified, it climbed straight up. Then, its high
pressure nearly exhausted, it turned leisurely and p