But, for the time being, the condition of the safe did not occupy any place in Jack’s thoughts. His sole care was for his father. Hastening to the house at top speed, he found that Professor Chadwick had been placed in bed and a physician summoned.

The doctor was coming out of the room just as Jack, with a pale, agitated face, came flying in.

“Oh! Dr. Goodenough,” he exclaimed, “how is dad? What has happened?”

“Be calm, my lad,” said the doctor kindly, placing a hand on the excited boy’s shoulder. “Your father has suffered nothing worse than an attack of vertigo brought on by overwork and study. A few weeks’ quiet will make him perfectly well again, and then I shall forbid him overexerting himself.”

“Can I see him?” asked Jack eagerly.

“Not just now. He is still only partly conscious. From what I can gather, the servant who answered the bell found him lying on the floor of the library unconscious. He was carried to his room, and I was sent for at once.”

“When can I see him?” demanded Jack anxiously, and Tom, who had now arrived, repeated the question.

“Probably this evening, when I shall pay another visit.”

“He is only suffering from vertigo, doctor?” asked Jack, with curious insistence, “not from any—any injuries?”

“Injuries? I don’t understand you.”

“He had not been in any struggle, then? That’s what I mean.”

“Of course not. What an odd question!” The doctor looked at Jack quizzically. “I shall have you under my care next,” he said jokingly.

“I thought that perhaps——”

Jack hesitated.

“Go on, my lad. I can see there is something on your mind. What is it?”

“Just this, doctor. Old Jupe, our colored man, told me that the door of the library safe, in which some valuable papers were deposited, was open when my father was found.”

The doctor’s face grew serious.

“I knew nothing of this,” he said. “Were there any signs that a violent entry had been effected?”

“That I don’t know, doctor. Naturally I came here first to find out my father’s condition.”

“It need give you no worry, my boy. I can assure you of that. Let us go to the library at once. What you have just told me may place a very different light on the matter.” And the doctor’s face grew serious.

“How is your father, Jack?”

Jack turned, and saw Ned Nevins, who was, by this time, one of the household, at his elbow. The boy’s face was troubled, for he had a genuine affection and regard for the good Professor.

“He has simply had a stroke of vertigo. It is nothing serious, Dr. Goodenough says. But, Ned, the safe——”

“I know. I heard what Jupe said.”

“Ned, the papers—your papers—may be stolen. How can I——”

“Say nothing about it, Jack. So long as your father has not been injured I do not care. Do you think that gang of rascals would have dared to break in here?”

“We can’t tell anything till we have examined the library. We are going there now. Come along.”

In the library everything was in order. The servant who had answered the bell was summoned and declared that things were exactly as they were when she replied to the Professor’s summons. He was lying at the foot of a desk when she entered the room and was quite unconscious.

“Let us examine the safe,” said Dr. Goodenough.

The door of the safe was ajar, and the servant declared that it had not been touched by any one since the discovery of the Professor’s unconscious form.

“You are quite certain of this?” asked the doctor.

“Oh, yes, sir. Positive.”

“Jack, where were the papers put?”

“In a drawer inside the safe, doctor.”

The boy had swung the door of the safe open, and the next instant he turned a white, startled face on the others.

“The drawer is empty. It has been robbed!” he exclaimed excitedly.

“Keep cool, my boy,” admonished the doctor. “You are sure the safe was closed when you left?”

“I shut it myself, doctor. There is not a chance that I could be mistaken.”

“And the combination?”

“I gave it to my father with my own hands. It was the last thing I did before I left.”

“Then the safe could only have been forced open unless some one possessing the combination opened it.”

“That is the only way any one could have gained access to its contents.”

“And yet there is not the slightest evidence that these doors have been forced,” said the doctor, who had been examining the safe. “This is a most mysterious occurrence.”

“How could the robbers have opened it?” demanded Jack.

“How did they get in, anyhow?” Tom wanted to know. The boy had been looking about the room. “This window is closed and locked with a snap-lock on the inside. Uncle must have felt chilly and closed it, or was it shut when you left, Jack?”

“It was shut,” said Jack positively. “I recollect that, because I asked dad if he didn’t want it closed, and he asked me to shut it.”

“There’s soft mould in the flower bed outside,” struck in Ned. “If any one had come in that way they must have left their footprints on the dirt.”

“That is so,” agreed the doctor. “Let us look at the ground outside the window.”

But an examination of the flower bed only deepened the mystery. It was a bed about five feet wide, and there was no possibility of any one’s having stepped across it without leaving the imprint of his feet. It had rained two days before, too, so that the ground was moist and would have readily retained any impression.

Yet there was not the slightest trace of a footprint to be seen. The little group exchanged puzzled glances.

“Perhaps somebody got in by the front door,” suggested Jack, but on inquiry it was learned that Jupe had been busy polishing floors in the front part of the house most of the day, and nobody could have got past without being seen. The only other entrance to the house was by the kitchen, and the cook was certain that nobody had come in through her domain.

As a last resort they examined the scuttle on the roof. It locked on the inside, and the fastenings had not been tampered with. Completely nonplussed, the investigators halted and talked matters over. Dr. Goodenough eventually decided to question Professor Chadwick that evening if he should be strong enough.

As may be imagined, the mystery of the theft of the papers cast a gloom over the household. Jack felt that he was partly responsible, and said so to Ned Nevins. But the latter indignantly bade him to say nothing about it.

“Let us be glad that the robbers did not injure your father,” he said. “The plans are gone and that is all there is to it.”

“But if they are not recovered, Ned, how can we ever make restitution to you?”

“If they are not recovered we still have the Electric Monarch. We must hurry and draw up another set of plans based upon her structure and rush them through the patent office.”

“That’s about the only thing to do,” agreed Jack ruefully, “but I can’t tell you how bad I feel, Ned, over the loss of your property which you entrusted to our care.”

“Forget it,” said Ned boyishly, and, although the expression was slangish, it conveyed to Jack a sense of consolation, for he felt that Ned would never blame him for the loss of Jeptha Nevins’s lifework.