McGuire grinned at the captain as they took up the ear-phones. "Power of suggestion," he whispered, but the smile was erased from his lips as he listened. For in his ear was sounding a weird and wailing note. No dots or dashes, as the operator had said, but the signal was strong. It rose and fell and wavered into shrill tremolos, a ghostly, unearthly sound, and it kept on and on in a shrill despairing wail. Abruptly it stopped. The captain would have removed the receiver from his ear, but the operator stopped him. "Listen," he said, "to the answer." * * * * * There was silence, broken only by an occasional hiss and crackle of some far distant mountain storm. Then, faint as a whisper, came an answering, whistling breath. It, too, trembled and quavered. It went up--up--to the limit of hearing; then slid down the scale to catch and tremble and again ascend in endless unvarying ups and downs of sound. It was another unbroken, unceasing, but always changing vibration. "What in thunder is that?" Captain Blake demanded. "Communication of some sort, I should say," McGuire said slowly, and he caught the operator's eyes upon him in silent agreement. "No letters," Blake objected; "no breaks; just that screech." He listened again. "Darned if it doesn't almost seem to say something," he admitted. "When did you first hear this?" he demanded of the radio man. "Night before last, sir. I did not report it. It seemed too--too--" "Quite so," said Captain Blake in understanding, "but it is some form of broadcasting on a variable wave; though how a thing like that can make sense--" "They talk back and forth," said the operator; "all night, most. Notice the loud one and the faint one; two stations sending and answering." Captain Blake waved him to silence. "Wait--wait!" he ordered. "It's growing louder!" * * * * * In the ears of the listening men the noise dropped to a loud grumble; rose to a piercing shriek; wavered and leaped rapidly from note to note. It was increasing; rushing upon them with unbearable sound. The sense of something approaching, driving toward them swiftly, was strong upon Lieutenant McGuire. He tore the head-phones from his ears and rushed to the door. The captain was beside him. Whoever--whatever--was sending that mysterious signal was coming near--but was that nearness a matter of miles or of thousands of miles? They stared at the stormy night sky above. A moon was glowing faintly behind scudding clouds, and the gray-black of flying shadows formed an opening as they watched, a wind-blown opening like a doorway to the infinity beyond, where, blocking out the stars, was a something that brought a breath-catching shout from the watching men.