If he never did so again, Sandy lived up to his decision to turn over a new leaf for once.

Usually impulsive, generally quick to adopt any new suspicion, he surprised his chums by catching Larry by the coat and dragging him back to the ground as his foot rested on the wing-step bracing.

“No!” he cried. “No! Larry—Dick—you, Mister! Come on, quick—under these trees yonder!”

They stared at him.

“Don’t you understand?” he urged. “Jeff will fly over his crate to see if it’s all right. He may see us. Come on!”

So sound was his argument that the others hurried with him to the concealment of the nearby grove, after Larry had thoughtfully cut out the ignition so that the propeller would not revolve if its observers flew low enough to distinguish its position.
205

Well hidden, they learned how wise Sandy had been.

Coming closer as it dropped lower, the amphibian circled in a tight swing over the fairway several times and finally straightened out, flying toward the wind that came from almost due North on this first cool day after a humid July week, and began to grow smaller to the watchers.

“We’d better get that engine started, now.” Dick left the grove.

“Let’s be careful,” commented Sandy. “They may come back.”

“We can be warming it up and watching!” Larry urged.

“We don’t need to hurry,” Sandy insisted. “I think I know—at last!—what this all means.”

Three voices, that of the caretaker no longer grumpy, urged him to explain. Too earnest to be proud of his deductions, Sandy spoke.

“When the hangar was first haunted, and we found chewing gum that the ghost had put there, as we thought,” he told an interested trio, “none of us could work out any answer to the puzzle.
206

“But stop and think of these things,” he continued, urging his two friends to use their own imaginations. “The amphibian was old-looking and didn’t seem to be much good, and the gas gauge was broken, and the chewing gum was quite fresh. That might look as though——”

“Some pilot was getting the ‘phib’ ready to fly and chewed gum as he worked and put the gauge out of order to keep anybody from knowing he had filled the gas tanks.”

“Good guess, Larry! It’s the way I work it out,” Dick added.

“Go on, young feller.” The caretaker was absorbed.

“Well,” Sandy grinned, “the chewing gum disappeared! Supposing the fellow we thought we saw vanishing really was there and got out some way. He’d know, from Jeff landing us and our going in, that the amphibian might not be usable when he’d need it——”

“So he went back and got the gum—but why?”

“He was getting that ready, Dick, for the emeralds—remember how Sandy discovered the place the imitations were hidden?”

“That’s so, Larry. Go on, Sandy. You’ve got a brilliant brain!”

“Oh, no,” Sandy protested. “It just flashed over me—putting all the facts together, the way I made up my mind I’d do.”

He outlined the rest of his inference.
207

“That was proved—the seaplane coming out to the yacht proved that the passenger who said he was a London agent, and wasn’t at all, had changed his plans. Well, say that he had arranged with Mimi, Mrs. Everdail’s maid, to have her throw over the jewels——”

“But she wouldn’t make the mistake of giving a confederate the wrong ones. She’d seen the real ones.”

They were working on the check-up and warming of the engine as they talked. Dick made the objection to Sandy’s theory.

“She’d know that the man knew the difference too!” Larry added.

That could be true, Sandy admitted. But he argued that the girl must have seen the captain take the stern life preserver to his cabin, and might have guessed, even observed through a cabin port, what he did. In that case she would have thrown over the life preserver knowing that her confederate would put it in the seaplane. And he had done exactly that!

“But the passenger jumped with a different life preserver!” Dick was more anxious to prove every step of Sandy’s argument than to find flaws in it.
208

“I think we found the life preserver that they might have had on board the seaplane all the time. And the other one—we never thought of the yacht’s name being painted on its own things. So we took it for granted that we had the real hiding place.”

“You argue ............