"How do you know?" asked Mr. Sumner, while Hal p.r.i.c.ked up his ears.

"Here are the marks made by a knife blade. They are quite fresh."

"Here are the marks of foot-prints upon the window-sill," added Hamington. "See, they lead toward that door yonder. Where does that door lead to?"

"An alley-way," replied Mr. Allen.

"Opening on the next street?"

"Yes."

"Then these marks add a new feature to the case," said Parker. "Has anybody been through the window to your knowledge?"

Every one shook his head.

"Queer," said Mr. Sumner. "If the thief came through the window how did he know the combination of the safe?"

"I have it," said Hamington. "You often opened the safe during the daytime, did you not?"

"Yes."

"Then the would-be thief watched you from the window. By looking from the left side he could easily see you work the combination without being seen himself. He watched you until he was sure he had the combination down fine, and last night he opened the window, stepped inside, opened the safe and took out the tin box, closed the door again, and escaped as he had come."

CHAPTER VI.

A BLOW IN THE DARK.

Was Hamington's explanation the correct one?

"By Jove! I believe that's the straight of it!" exclaimed Hardwick.

"So do I," said Mr. Allen. "It is the only way to account for the marks on the window-frame and the sash."

Mr. Sumner said nothing. Indeed, to tell the truth, his loss had set his mind in a whirl.

Parker turned to Hal.

"Did you open the office this morning?" he asked.

"Yes, sir."

"You opened the window when you cleaned up?"

"Yes."

"Was it locked?"

Hal tried to think.

"I believe it was."

"You are not sure?"

"No, sir."

"He ought to be," broke in Mr. Allen. "I would know, if I was in his place."

"See, if you cannot think, Hal," said Mr. Sumner.

"If anything, I think the window was locked," said the youth, candidly.

"You do?" said Parker.

"Yes, for I believe I had some trouble to unfasten it."

The two detectives looked at each other.

"If that is so, it alters the case," said Hamington. "But I believe the boy is mistaken."

"So do I," added Hardwick, promptly.

Parker opened the window again, and leaping out, made his way to the alley. Hamington went after him. Then several customers came in, and Mr.

Allen and Hardwick went forward to wait upon them.

It was a quarter of an hour before the detectives came back, and then they had very little to say, excepting that they would report the particulars at headquarters and endeavor to run down the criminal.

Mr. Sumner was broken down by his loss. He sat in his private office nearly all of the remainder of the day, his head resting in his hands.

Mr. Allen went off on business, and Hardwick stuck to his books as if his life depended on it.

Hal resumed his duties with a heavy weight on his heart. For some reason he had expected to be discharged, but nothing was said about his leaving.

Hardwick scowled at the youth every time their eyes met, and kept piling the work upon Hal. The book-keeper was nervous, and the youth did not fail to notice this, and it set him to thinking.

If only he had listened more attentively to what had been said on the ferry-boat that night! Hal was sure if he had done this he would have known if Hardwick and Allen were guilty or not.

Then Hal began to speculate on the foot-marks on the window-sill. If the thief had entered the office that way, why were not some of the same marks visible on the carpet in front of the safe?

When Hardwick went out to lunch, Hal watched him from the office window.

At the corner he saw the book-keeper joined by d.i.c.k Ferris, and the two seemed to be in earnest conversation as they walked along.

When Hardwick came back Hal was given a half hour. The boy put on his hat and coat and went out. He did not feel like eating, and he walked up to the corner and around to the back street, intending to pay a visit to the alley through which the robber was supposed to have escaped.

Just as he was about to turn into the narrow place, now piled high with snow, somebody caught him by the shoulder. Turning, he found himself confronted by d.i.c.k Ferris.