With the high resistance receiving apparatus over his head, Arnold was listening to the wireless signals that came over his "radio detective" on the yacht, moving the slider back and forth on a sort of tuning coil, as he listened. Woodward stood close beside him.
"As you know," Arnold remarked, "by the use of an aerial, messages may be easily received from any number of stations. Laws, rules, and regulations may be adopted by the government to shut out interlopers and to plug busybody ears, but the greater part of whatever is transmitted by the Hertzian waves can be snatched down by this wireless detective of mine. Here I can sit in my wireless room with this ear-phone clamped over my head drinking in news, plucking the secrets of others from the sky—in other words, this is eavesdropping by a wireless wire-tapper."
"Are you getting anything now?" asked Woodward.
Arnold nodded, as he seized a pencil and started to write. The lieutenant bent forward in tense interest. Finally Arnold read what he had written and with a peculiar, quiet smile handed it over. Woodward read. It was a senseless jumble of dots and dashes of the Morse code but, although he was familiar with the code, he could make nothing out of it.
"It's the Morse code all right," he said, handing it back with a puzzled look, "but it doesn't make any sense."
Arnold smiled again, took the paper, and without a word wrote on it some more. Then he handed it back to Woodward. "An old trick," he said. "Reverse the dots and dashes and see what you get."
Woodward looked at it, as Arnold had reversed it and his face lighted up.
"Harbor successfully mined," he quoted in surprise.
"I'll show you another thing about this radio detective of mine," went on Arnold energetically. "It's not only a wave length measurer, but by a process of my own I can determine approximately the distance between the sending and the receiving points of a message."
He attached another, smaller machine to the wireless detector. In the face was a moving finger which swung over a dial marked off in miles from one upward. As Arnold adjusted the new detector, the hand began to move slowly. Woodward looked eagerly. It did not move far, but came to rest above the figure "2."