t time, Sarka caught up cube after cube and hurled them
all after the first.

* * * * *

Out of the crater there came no sound of heavy objects striking,
though Sarka felt there should have, for the cubes were almost as
heavy as a man.

Then his hair almost stood on end under his helmet, for under that
first aircar, where he had first seen it, the initial cube was again
gleaming into life!

The thing had dissolved while being hurled over the rim, and reformed
in its proper place, its station as silent sentinel under the aircar!

These cubes then, were indeed sentinels--sentinels impossible to
injure. Though no force had been used against Sarka and Jaska, Sarka
had the feeling that they were powerless, and that here on the edge of
a crater of the Moon awful forces were being mustered against them.
Mustered slowly, sluggishly, yet surely, as though the mentality which
mustered them knew them helpless, and that there was no need to hurry!

As for Jaska, she merely clung to Sarka and waited--trusting him no
matter what might transpire.

On a blind chance, Sarka brought out his ray director again, turned
its muzzle toward that invisibly-blue column, pressed with his
fingers, moving the director back and forth.

Instantly the blue column seemed to break short off, while the broken
upper portion started racing outward toward the Earth. Sarka watched
it, and noted that the yellowish glow on the Earth, even as he
watched, was fading out--disappearing!

"If the ray will smash the blue column, Jaska," he said, "it will also
destroy its source! Come! We will go look for it!"

And, holding her hand tightly, he rose to his feet and strode boldly
down the inner slope of the vast crater.

CHAPTER XIV

_The Crater Gnomes_

It seemed to Sarka, as he moved down the inner slope of the crater,
that the cubes were somehow making sport of him, laughing at him,
though no hint of laughter or anything resembling laughter emanated
from them.

But, shutting his lips grimly, holding fast to Jaska's hand, he
proceeded on, reached the lower portion of the inner slope, where it
dropped off into a seeming black abyss, and dropped, keeping to a safe
speed because of the fact that both he and Jaska were attired for
movement in the air--though their manner of aerial transportation
could scarcely be called flying.

The anti-gravitational ovoids simply rendered ineffectual the law of
gravity.

Down the