"What time do you think the attack will occur?" Bud asked.
Tom shrugged. "The 'twenty-four-hour' business may have been approximate. But I'd say from two o'clock on is the danger period."
The young inventor checked frequently with Washington and the other crews stationed around the country. Suddenly the radiotelephone operator gave a yell.
"Your father is on the line, skipper!"
The scientist was calling from the receiver-computer headquarters at Enterprises. "Exman has reported a quake pulse will be sent in seven minutes—at 21.36 G.M.T."
"I'm ready, Dad," Tom said, then asked for various technical details before hanging up.
He passed the word to the crew and glanced at his watch. A hasty, last-moment inspection was carried out, every man checking certain details of the setup.
Soon the pulsemakers began ticking inside the dual-control spheres as they picked up the frequency signal by radio. Tom studied the gauge dials.
Tension mounted rapidly among the waiting group. The same thought was throbbing through every mind:
Was the nation on the brink of a terrible disaster? Or would Tom Swift's invention safeguard the threatened area?