urried across the street to the empty lot and the
three cadets followed.

"Take it easy, Astro," said Tom, when he saw the big Venusian gripping
his fists in frustration. "Anyone could make a mistake."

"That's just it," said Astro. "I'm not mistaken! Those jokers must have
changed over from chemical fuel to reactant drive!"

"But why?" asked Roger. "That would cost more than they could make in
ten years of hauling passengers on joy rides!"

Astro whirled around and faced the two cadets. "I'm telling you the ship
that blasted off from here was an atomic drive. I don't know any more
than that, but I _do_ know that!"

There was a sudden shout from Strong and the three boys hurried to the
shack. The Solar Guard captain and the exposition commissioner were
standing inside and playing the beam of an electric torch around the
walls.

"Looks as though you were right about the atomic drive, Astro," said
Strong. He flashed the light into one corner where a tangled jumble of
lines lay on the floor. "That's feed-line gear for a chemical burner,
and over there"--he played the light on some empty cartons--"is what's
left of the crate's lead baffling it shipped in. They must have changed
over to atomic drive recently."

Astro accepted the statement with a nod. It wasn't in the nature of the
big cadet to boast. Now that the secret of the ship had been resolved,
he turned, like the others, to the question of why?

"I think the best thing we can do," said Strong, "is to spread out and
search the whole area. Might find something to indicate where they
went." Commissioner Hawks nodded his head in agreement.

While Tom, Roger, and Astro searched outside, Strong and Hawks went
through the drawers of the dusty desk standing in one corner.

"Nothing here but a record of the flights they made, bills for chemical
fuel delivered, and the like," said Hawks at last. "They were losing
money on the operation, too. Think they might have just gotten fed up
and pulled out?"

Strong was rummaging around in one corner of the shack. "I'd go along
with that, but for one thing, Mike," he said. "Take a look at this." He
held up a small cloth bag. "There's dirt in the bottom of this bag. And
there are about fifty more bags in that corner."

"Dirt!" exclaimed the commissioner.

"Yep," said Strong grimly. "So we found out who was dumping the dirt.
But we still haven't found out why."

"Or where it came from," said Hawks.

Strong tossed the ba