and biologists, anyway. The
engines were their main interest, huge things which overshadowed
everything about them.

It must have been the concealment afforded by the engines that permitted
three of the enemy to get so close. The only warning the Terrestrians
had was a faint pink haze as they stepped around the corner of an
engine; and a sudden feeling of faintness swept over them. They leaped
back, out of sight, peering around the corner with nerves and muscles
tensed. There was no sign of movement.

As they watched, they saw a pallid hand reaching out with a ray gun; and
Wade swiftly pointed his own weapon. There came a sudden crash of metal,
a groan and quiet. Two other aliens leaped from behind the great engine
just as the Terrestrians dodged further back; as swiftly, they too found
concealment.

Arcot swung his ray up, and was about to pull the trigger that would
send the huge engine toppling over upon them, when he saw that it was
running. He thought of the unknown energies in the machine, the
potential destruction, and he shook his head. Cautiously he looked
around the edge of the towering mass, waiting--his beam flashed out, and
there was a snapping sound as the ray caught a reaching hand and hurled
its owner against a mighty transformer of some sort. For an instant the
huge mass tottered, then was still. In the low concentration of power
that Arcot had used, only a small portion had been touched, and the
molecules of this portion had not been enough to tip over its tremendous
weight.

Only one enemy remained; and Arcot learned swiftly that he was still in
action, for before he could dodge back there came that now-familiar pink
haziness. It touched Arcot's hand, outstretched as it had been when he
fired, and a sudden numbness came over it. His pistol hand seemed to
lose all feeling of warmth or cold. It was there; he could still feel
the weapon's deadened weight. Reflex action hurled him back, his hand
out of range of the ray. In seconds feeling began to return, and in less
than ten his hand was normal again.

He turned to the others with a wry grin. "Whew--that was a narrow
squeak! I must say their ray is a gentlemenly sort of thing. It either
kills you, or doesn't injure you at all. There it goes again!"

A shaft of pink radiance reached the end of the engine, just grazing it,
evidently absorbed by its mass. "Pinning us down," Wade grated. They
certainly couldn't step out into the open space--but they