rking
on?"

"Yes," Dixon answered grimly. "I have an explosive right here in the
laboratory that can easily blow the Centaurian's camp completely off
the map."

* * * * *

Crawford shook his head impatiently. "Destroying the camp would do no
good. We must shatter the space ship itself if we are to extinguish
those green rays in time to save our world."

"That is impossible if the space ship is hovering up there by the
moon!" Dixon protested.

"No, it is not impossible," Crawford answered confidently. "I have a
projectile in my laboratory that will not only hurtle across that
great gap with incredible speed, but will also infallibly strike its
target when it gets there. It is a projectile that is as irresistibly
drawn by radio waves as steel is by a magnet, and it will speed as
straight to the source of those waves as a bit of steel will to the
magnet.

"The Centaurians in the space ship," Crawford continued, "are in
constant communication with their camp through radio apparatus much
like our own. If you can pack a powerful contact charge of your
explosive in my projectile, I can guarantee that when the projectile
is released it will flash out into space and score a direct hit
against the walls of the space ship."

"I can pack the explosive in the projectile, all right," Dixon
answered grimly. "We will need only a lump the size of an egg, and a
small container of the heavy gas that activates it. The explosive
itself is a radium compound that, when allowed to come in contact with
the activating gas, becomes so unstable that any sharp blow will set
it off in an explosion that in a matter of seconds releases the
infinite quantities of energy usually released by radium over a period
of at least twelve hundred years. The cataclysmic force of that
explosion should be enough to wreck a small planet."

"Good!" Crawford commended weakly. "If you can only strike your blow
to-night, Bruce, our world still has a chance. If only you--" The old
man's voice suddenly failed. He sank back in utter collapse, his eyes
closed and his last vestige of strength spent.

* * * * *

Knowing that the old man would probably remain in his sleep of
complete exhaustion for hours, Dixon turned his attention to Ruth. To
his surprise, he found her sitting up, apparently completely
recovered.

"I'm quite all right again," she said reassuringly. "I've been
listening to what Uncl