urveyed it thoughtfully.

"There must be a hidden spring somewhere," he said.

He walked carelessly along the wall, tapping it idly here and there.
His quick probing fingers were searching.

With a sharp "Ah!" he stopped short. He bent over a moment; his
fingers moved deftly. Then he straightened with a grunt of
satisfaction. A section of the seemingly solid, immovable stone was
sliding silently open. He looked through.

* * * * *

Nona saw him jerk his head back, heard his involuntary cry of horror.
Then she heard another cry: an excited warning shout. She whirled
around in time to see a Ganymedan running toward them from behind. A
deadly pencil-ray pointed straight at her companion. Without a
moment's hesitation she sprang at Grant, pushed him violently so that
he staggered and fell through the opening to the other side. In so
doing, she tripped over his body, and fell prone. That saved her life,
for a blue flame sheared clean through the stone, inches above her
head.

Grant squirmed around underneath. The electro-gun was somehow out of
the side flap and now it spat its explosive hail. The tiny bullets
flared into little puff balls of flame against the space-suit of the
Ganymedan. A long howl of anguish came to them as he threw up his
hands and fell into a shapeless heap. But a moment later there were
other cries, angry shouts. Pemberton was on his feet again with the
quickness of a cat. He pulled Nona up after him, thrust her to one
side, behind the protection of the wall. His eyes were blazing now,
aflame with the ardor of battle. Very carefully he leaned out and
pressed the trigger. The surging mob was caught in full flight. The
electro-bullets spread fanwise, exploded into flaming deaths. The
Ganymedans went down as though a huge scythe had swept through their
ranks. The survivors scattered hastily, throwing themselves headlong
to the surface of the rock to escape further execution.

"That'll hold them for a while," Grant laughed grimly.

"Drop your gun, and turn around--both of you." A cold, smooth voice
spoke in deadly menace directly behind them--a voice that came from
the mysterious inner side of the wall.

Grant spun around, his gun ready to fire. A ray snapped out at him, a
ray with a greenish tinge. The fingers of his gun hand grew suddenly
nerveless; the weapon dropped unresistingly from his paralyzed hand.

A tall Ganymedan towered before him, unhidden by a space