Tom was jubilant. “Great news, HarlanI Now keep your fingers crossed and wish us luck!”
Next day the Rockwell left its pier and steamed slowly out of the harbor. The Sea Hound shadowed it down the bay and out to sea. As the afternoon wore on, Tom and his crew kept a close watch with their sonar gear and hydrophones for any sign of an enemy sub.
Night fell. Shortly before ten o’clock, Arv Hanson, who was monitoring the hydrophones, called out, “Skipper, it just started all of a sudden! This must be about the time the bomb was supposed to go off.”
Arv flicked a switch. From the hydrophone loud-speaker came a steady beat: Plonk! … Plonk! … Plonk! … Plonk!
“That’s a sonar pulse!” Tom exclaimed. “Where’s it coming from, Arv?”
“Dead ahead. Must be from the Rockwell.”
Tom frowned in puzzlement, then his face
SONAR SIGNAL 159
clouded with dismay. “Vaxilis may have outsmarted us!”
“How do you figure that?” Hank Sterling asked.
“If I’m right, he learned that his bomb was disarmed and thought up another scheme. That pulse is a signal to help the fiends track the Rockwell. An enemy frogman must have attached a signaling device and a delayed-action bomb to the ship’s hull while she was moored back in port!”
“But I thought the Navy was keeping an underwater watch?” Arv objected.
“They were,” Tom replied. “But they probably secured the watch-I mean, called in their frogmen-after they nipped that bomb attempt in Baltimore. I believe that a bomb will blow a hole in the Rockwell’s hull sometime tonight!”
Suddenly the sonarman sang out, “Blip on the port quarter, skipper! It’s up topside!”
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