egged his son. "I'll do my best to recover those
things for you."

Several days passed, but there was no clew to the thieves. That they
were the same ones who had stolen the turbine model there was little
doubt, but they seemed to have covered their tracks well. The police
were at a loss, and, though Tom and Mr. Sharp cruised about the lake,
they could get no trace of the men. The balloonist had sent to
Pratonia for his clothing and other baggage and was now installed in
the Swift home, where he was invited to stay a week or two.

One night when he was looking over some papers he had taken from his
trunk the balloonist came over to where Tom was making a drawing of a
new machine he was planning and said:

"Like to see my idea for an airship? Different from some. It's a
dirigible balloon with an aeroplane front and rear to steer and balance
it in big winds. It would be a winner, only for one thing. Maybe you
can help me."

"Maybe I can," agreed Tom, who was at once interested.

"We ought to be able to do something. Look at our names--Swift and
Sharp--quick and penetrating--a good firm to build airships," and he
laughed genially. "Shall we do it?"

"I'm willing," agreed Tom, and the balloonist spread his plans out on
the table, he and the young inventor soon being deep in a discussion of
them.

CHAPTER XXIV

THE MYSTERY SOLVED

From then on, for several days, the young inventor and his new friend
lived in an atmosphere of airships. They talked them from morning
until night, and even Mr. Swift, much as he was exercised over his
loss, took part in the discussions.

In the meanwhile efforts had not ceased to locate the robbers and
recover the stolen goods, but so far without success.

One afternoon, about two weeks after the thrilling rescue of John
Sharp, Tom said to the balloonist:

"Wouldn't you like to come for a ride in the motor-boat? Maybe it will
help us to solve the puzzle of the airship. We'll take a trip across
and up the opposite shore."

"Good idea," commented Mr. Sharp. "Fine day for a sail. Come on.
Blow the cobwebs from our brains."

Mr. Swift declined an invitation to accompany them, as he said he would
stay home and try to straighten out his affairs, which were somewhat
muddled by the robbery.

Out over the blue waters of Lake Carlopa shot the ARROW. It was making
only moderate speed, as Tom was in no hurry, and he knew his engine
would last longer if not forced