Earthquake activity, Tom knew, tends to occur in circular patterns, like
bands around the earth--for instance, the circum-Pacific belt, and
another belt extending eastward from the Mediterranean through Asia and
on into the East Indies. Often these quake lines are visible as breaks
or ruptures along the ground surface, called _fault traces_. No doubt,
Tom thought, there were many more uncharted ones.
Could an enemy scientist be making use of these earth faults to produce
a man-made quake? Tom mulled over the disturbing idea.
"How would I tackle the job myself, if I had to undertake such a project
for national defense?" the young inventor mused. He felt a growing sense
of excitement as an idea began to take shape in his mind.
_What about an artificial shock wave!_
An hour later Bud Barclay walked into the laboratory and found Tom
hunched over a jumbled pile of reference books on his workbench.
"What cooks, skipper?" Bud asked.
Tom looked up, his blue eyes blazing. "Bud, I think I may have the
answer!"
Tom got up from his stool and paced about the laboratory. "Suppose the
Brungarian rebel scientists have invented some sort of shock-wave
producer--a device for sending vibrations through the earth's crust or
the mantle underneath."
"Okay, suppose they have," Bud replied.
Tom snatched up a piece of chalk and made some quick diagrams on a
blackboard. "Just this, pal.
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