For the Secret Service man confided that Inventor Bender’s sound detector had succeeded beyond his expectations and, incidentally, had entirely upset his previous calculations by what it revealed. He had believed, as earlier in San Francisco he had told them, that the smugglers’ cove was somewhere near San Diego in all probability. But the sound detector very definitely had located it as in the group of wild islands off Santa Barbara.
“Those are the islands,” he said, pointing to three mountainous formations rising from the sea to starboard. “Wild, craggy, isolated and large; sparsely inhabited, and not on any steamer track; not a town nor even a hamlet on any of them.
“They lie along this coast in a chain stretching seventy miles. There are three large ones, San Miguel, the most northern, Santa Rosa and Santa Cruz—the latter the largest and wildest. Then beyond Santa Cruz on the south lies the mysterious vanishing island, Anacapa. At high tide it is a group of little islands, almost submerged. At low tide, sandpits connecting the low hummocks are revealed. This gives it the name of ‘Vanishing Island.’”
“And is it on ‘Vanishing Island’ you believe the smugglers are located?” asked Frank.
“No, that island is practically uninhabitable, and, besides, would be too open to observation. It is on one of the other three, although which has yet to be determined. Good as is Inventor Bender’s sound detector, he said he was unable to locate the smugglers’ secret radio station more accurately than to say it was somewhere in that group. He gave me his reasons, but I know so little about radio that I could not follow him well.”
“I know a bit about the subject of sound detectors,” said Jack. “Although it was not generally known, radio compasses were employed by our forces and by the Allies, too, during the closing years of the war to locate sounds. However, such compasses were not very accurate, and from Inventor Bender’s description of his own improved device I received the impression that he had made a great advance.
“Probably,” he continued, “Inventor Bender could obtain almost the exact location of the smugglers’ radio if he were able to surround the suspected area with sound detectors. The detectors, then, would hunt out the exact hub where the secret radio was in operation. But, if he has brought only one or two detectors into play——”