.
"Adjustable, see? Unwind in their fall ... set 'em for any length of
travel ... fires the charge in the air. That's how they wiped out our
air fleet."

There were others without the propellers; they had fins to hold them
nose downward. On each nose was a small rounded cap.

"Detonators of some sort," said MacGregor. "We've got to have one. We
must get it out quick; the tide's coming in." He laid his hands upon one
of the slim, egg-shaped things. He lifted, then strained mightily. But
the object did not rise; it only rolled sluggishly.

The scientist stared at it amazed. "Specific gravity," he exclaimed,
"beyond anything known! There's nothing on earth ... there is no such
substance ... no form of matter...." His eyes were incredulous.

"Lots to learn," Thurston answered grimly. "We've yet to learn how to
fight off the other four."

The other nodded. "Here's the secret," he said. "These shells liberate
the same gas that drives the machine. Solve one and we solve both--then
we learn how to combat it. But how to remove it--that is the problem.
You and I can never lift this out of here."

His glance darted about. There was a small door in the metal beam. The
groove in which the shells were placed led to it; it was a port for
launching the projectiles. He moved it, opened it. A dash of spray
struck him in the face. He glanced inquiringly at his companion.

"Dare we do it?" he asked. "Slide one of them out?"

Each man looked long into the eyes of the other. Was this, then, the end
of their terrible night? One shell to be dropped--then a bursting
volcano to blast them to eternity....

"The boys in the planes risked it," said Thurston quietly. "They got
theirs." He stopped for a broken fragment of steel. "Try one with a fan
on; it hasn't a detonator."

The men pried at the slim thing. It slid slowly toward the open port.
One heave and it balanced on the edge, then vanished abruptly. The spray
was cold on their faces. They breathed heavily with the realization that
they still lived.

* * * * *

There were days of horror that followed, horror tempered by a numbing
paralysis of all emotions. There were bodies by thousands to be heaped
in the pit where San Diego had stood, to be buried beneath countless
tons of debris and dirt. Trains brought an army of helpers; airplanes
came with doctors and nurses and the beginning of a mountain of
supplies. The need was there; it must be met. Yet