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 too frequently.  They glided along, crossed the lake and were proceeding up the opposite shore when, as they turned out from a little bay and rounded a point of land, Mr. Sharp exclaimed: 

“Look out, Tom, there’s rowboat just ahead!”

“Oh, I’ll pass well to one side of that,” answered the young inventor, looking at the craft.  As he did so, noting that there were four men in it, one of the occupants caught a glimpse of the arrow.  No sooner had he done so than he spoke to his companions, and they all turned to stare at Tom.  At first the lad could scarcely believe his eyes, but as he looked more intently he uttered a cry.

“There they are!”

“Who?” inquired Mr. Sharp.

“Those men—­the thieves!  We must catch them!”

Tom had spoken loudly, but even though the men in the rowboat did hear what he said, they would have realized without that that they were about to be pursued, for there was no mistaking the attitude of our hero.