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 fr