It
illustrates the fact that two things cannot occupy the same place
at the same time.
Pushing the tumbler, open end down, into the pail of water,
leaves a space in the upper end of the tumbler which the water
cannot fill, because it is already occupied with air. Imagine a
big tumbler, made of thick steel, lowered into the water. Air
pumped into the upper part not only keeps the water from
entering, but also enables a man inside to breathe and to move
about inside the bell which may be lowered to the floor of the
ocean. But, as Tom told Ned, his diving bell was a big
improvement over those commonly used.
The two young men inspected the progress made in refitting the
submarine, and Tom expressed himself as satisfied.
"How soon do you think you can start?" asked Ned.
"In about two weeks," was the answer. "I'll want to get to the
West Indies before the fall storms start. Not only will it be
impossible to make a search then, but the very location of the
sunken wreck may be changed."
"How so?" asked Ned.
"Because of undersea currents.
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