Dr. Goodenough’s visit that night did not serve to throw any light on the mystery of the safe robbery. In the meantime the police had been summoned, and investigated the premises without getting any nearer a solution of the puzzle than the boys and Doctor Goodenough had done.

Jack had taken it upon himself that afternoon before supper to telephone to the Hinkley House. He learned there that Sam was out and not expected back for some time. Heiny Dill was dispatched to the village to learn further particulars, and returned with the report that Hank Nevins and Miles Sharkey had both left the Hinkley House shortly before his arrival on the scene.

The young amateur detective had trailed them as far as the depot, only to find that they had taken the train bound south a few moments before he arrived. He had learned, however, that they had spent most of the day previous to their departure in the hotel. This only served to make matters the more baffling.

By common consent, whether justly or unjustly, the boys had been inclined to suspect either Hank or Miles with being concerned in the robbery. But it seemed that they had a complete alibi. Sam Hinkley, too, had been seen about the village on and off most of the day, and thus he also was eliminated. But the boys had not suspected Sam in the matter, anyway, so this information was not a surprise to them.

“This is a mystery for fair,” declared Jack, when Heiny Dill had duly reported the facts to him. “Fellows, we are stumped.”

“Possibly to-night your father will be able to talk and throw some light on the matter,” suggested Ned.

“Perhaps so. I am sure I hope that he will. A mystery like this gets on your nerves. The only people I can think of who knew of the existence of the plans, except ourselves, are Hank and his friend Sharkey. From what you say of them, Ned, I guess they wouldn’t stick at anything; from what Heiny Dill has found out we know it was impossible for them to be here at about the time of the robbery. Dad was found unconscious about an hour after we left. At that time Hank and his friend were in the village. They were seen there talking to Sam Hinkley.”

“If we could get hold of Sam maybe he could tell us something,” suggested Tom.

When Dr. Goodenough arrived that evening he informed Jack that Professor Chadwick had sufficiently recovered to be able to talk. With what eagerness they all awaited the outcome of that interview may be imagined. But so far as helping to clear up the mystery was concerned, Professor Chadwick was as powerless as any of them.

“After Jack had closed the window and left,” he said, “I lay down upon the lounge. After a time I felt better and thought I would get a book. I rose from the couch and went toward the bookcase. I can recollect nothing more till I found myself in bed with Dr. Goodenough in attendance on me.”

“Nothing else at all?” gently urged the doctor.

“Nothing except that Jupe came in to tell me that the Electric Monarch had started successfully on her maiden voyage.”

“You can recall nobody attempting to force the window or open the safe?”

Professor Chadwick shook his head positively.

“Nothing like that at all, doctor,” he said, with conviction.

“And nobody but Jupe entered the room, to your knowledge?”

“Nobody,” declared Professor Chadwick, “and I think we can safely leave Jupe out of the question.”

Late that night Jack called up the Hinkley House and discovered that Sam had not returned.

“I thought he was up to your place,” said Landlord Hinkley. “I’ve no idee whar’ the boy hes gone. He ain’t often out this late at night. I hope he ain’t up to any monkey shines. If he be, I’ll whale him good, big as he be.”

Jack decided that it was no use telling Sam’s father of all that had occurred since the morning. But when he hung up the receiver he was a sadly perplexed boy. When Heiny Dill departed for the hotel that night he promised to find out what he could. On his return the next morning he reported that a wire had been received from Sam, who said that he was going to New York. Landlord Hinkley found, incidentally, that the funds to finance Sam’s journey had been taken from his cash drawer. This was the sum total of young Dill’s information, and it was not enlightening.

In fact, it complicated the puzzle, for if Sam was not implicated in the robbery, and there was nothing to make them believe that he was, there was no apparent reason why he should decamp so suddenly, unless he feared that he might be prosecuted for the theft of the lever. The boys, therefore, were forced to conclude that this was the reason for Sam’s flight.

As for the sudden departure of Hank and Miles Sharkey, that was more understandable. They had practically hired Sam to make his desperate attempt to cripple the Electric Monarch, and knew that their plans must have been foiled when they saw the craft take to the air. This being so, they had probably argued that Sam would be arrested and would implicate them. Flight, then, must have seemed to them to be their wisest course.

And so, for the present, the mystery of the stolen plans had to be given up by the police and those most interested in the recovery of the papers, as an unsolvable puzzle. Of the startling way in which it was to be cleared up, none of those concerned had the slightest inkling. From day to day the boys feared to hear of the plans being filed in the patent office. But, although through Prof. Chadwick’s patent lawyers in Washington, they kept in constant touch with the National Capital, no such papers turned up. In the meantime the boys busied themselves making as complete a set of duplicate plans as possible, covering every patentable feature of the Electric Monarch.