t.

Outside, the Sun shone down brighter than it had before. The sky was
the calm serene blue of a cloudless day. Burl knew that at that same
moment, all over the world, the sky was clearer and the Sun warmer.

But for how long? Behind them the building still stood--and its
inventors were still to be found.

Chapter 3. _The Secret of A-G 17_

The Dennings did not have much time to speculate on the mystery of the
Sun-stealers. For just as they were discussing what should be their next
course of action, the problem was solved for them. There was a roaring
in the air, then a humming, and in a matter of a few more seconds, six
rocket helicopters popped into sight, hovered over the valley on
streaming jets, and settled down.

"They're U.S. planes!" gasped Burl, jumping to his feet and going to
meet them. "It must mean that they know we stopped the machines."

"Obviously," said his father, striding with him to greet the helmeted
man who was now stepping out of the lead machine. By this time the last
of the squad had landed, and the khaki-clad soldiers in them were
already disembarking. "I imagine that all over the world the sky turned
a little brighter. It must have been apparent at once."

The leader of the 'copter men reached them. He was a tall, bronzed man,
wearing the service coveralls and markings of a captain of the Air
Force. He stretched out his hand. "You must be the Dennings. I'm Captain
Saunders. I've been asked to bring you back with me right away so that
we can get a complete report on this affair. How fast can you get
ready?"

"Why," said Burl, "we're ready right now. As soon as we can dump our
packs aboard. But, gee, you mean go back--where?"

Saunders smiled grimly. "To California. We just left there. I have been
given urgent orders to waste no time. So will you oblige?"

The two Dennings looked at each other. This was important, all right.
They realized that these planes had flown on fast rockets the instant
the sky had cleared. Possibly there was still a crisis--one they had not
heard of.

They did not pause to ask further questions. Mark Denning asked the
captain to dispatch one of his 'copters to the camp beyond the mountains
to tell Gonzales to load up and start back for Lima. This order given,
the two Dennings climbed into the rocket 'copter, and Saunders took the
controls.

With a whoosh, the squat craft lifted on its rockets, its jet-driven fan
carried it up, folded, and the roc