“I’ll promise,” was the fellow’s laconic response.

“By the way,” Bud remarked, as they were about to leave the cabin, “would you mind telling us the handle of your name?  We know your father’s surname, but we’d like to know how to address you.  You’re too young for us to call you Mr. Howard.”

“You c’n call me Bill, if you want to,” the slingshot victim replied.

Hal was particularly impressed with a sly, cunning look in the eyes of the prisoner and told himself that the fellow would bear watching to keep him out of mischief.

“I tell you what I’d like to do,” he said to his two friends as they reached the deck.  “I’d like to hide in the closet in the cabin and watch that fellow.  I bet he’d do something that would help us break his mysterious silence.”

“You could steal down into that little alcove near the entrance of the cabin and watch him there through the crack in the door,” Bud suggested.

“That’s second best choice,” said Hal, “I think I’ll make use of it at once.”

Accordingly he descended the companionway with the greatest caution and succeeded in ensconcing himself in the position suggested by Bud.  He had not been there long when he was amply rewarded for his diligence.

He could hear the prisoner moving about in the cabin and a peep through the long narrow aperture along the hinge side of the door acquainted him with the object of the Canadian boy’s interest.  The latter, apparently, had just seated himself at the table, and with phones to his ears, was in the act of tuning the instrument.

Presently he appeared to be satisfied with this preliminary and put his hand on the sending key.  The fellow seemed to be perfectly at home with the outfit.  Now the key was tapping and the spark was leaping across the gap.  The secret watcher leaned forward eagerly to catch every sound.  Yes, it came in genuine enough dots and dashes, and he read them with ever increasing astonishment.

First the operator repeated a Canadian call several times.  Then, apparently, the call was acknowledged, and he sent the following message: 

“I am prisoner on yacht, Catwhisker, in hands of the fellows I tried to hold back, with radio, as they were leaving Oswego, N.Y.  They are determined to solve mystery of your doings.  Don’t bother about me, but tell pa to clean out his place as soon as possible and then let his prisoner go.  They have government officer with them on his trail and will soon find his hiding place and raid it.”

“My goodness!” Hal breathed excitedly.  “Now I’m getting at the bottom of this affair.  That boy is the anonymous amateur who pretended to have a radio wager with Hal’s cousin and tried to make us think his Sos was a joke.”

CHAPTER XXIX

The End of the “Mystery”

Hal almost held his breath in his eagerness to maintain perfect silence in order that he might “listen-in” to this radio transmission until the sender had telegraphed all that he had in mind to send.