“I’ll set the house detective on this,” he said.  “We can’t allow anybody to prowl around, trying to use false keys.”

“We want to catch that man ourselves,” said Dick.  “We are going to set a watch for him.  No more sleep for us to-night.”

“I don’t blame you,” returned the clerk.  “If you spot him, call up the office and we’ll give you all the help you want.”

The boys hurried into their clothing, and then, led by Dick, walked noiselessly through the various hallways of the big hotel in the direction where the oldest Rover boy had heard the door shut.  But though they passed many doors, Dick could not determine which was the right one.

“Let us set a regular watch,” suggested Tom “We can take turns.  One can watch while the others sleep.”

“All right, I’ll watch first,” answered Dick.

“Call me in an hour, Dick,” returned Tom.

“And call me an hour after that—­ if you want me,” added Sam.

The hallway was long and but dimly lighted.  At the end was a sofa, and after walking up and down several times, Dick sat down on this.  The long journey from Valley Brook farm had made him sleepy, but he resolved to keep wide awake, in case the mysterious individual should again show himself.

“He’s got to come out of his door some time, unless he tries to get away by a fire escape,” thought the youth.  “And I guess all the fire escapes on this side of the building are at the end of the hall.  I hope I’ve got him trapped, whoever he is.”

Half an hour went by and nothing unusual happened.  Then Dick heard a distant elevator stop, and two men got off and came down the hallway.  They stared rather curiously at the youth.

“What’s the matter?” asked one, presently.

“Waiting for a friend,” was the answer.

“Humph! rather late,” remarked the man.

“Better say early, Jack,” laughed the other.  “It’s ten after two.”

“Is that so!  Great Scott!  Time we got to bed!” And the two men passed into a nearby room, locking the door after them.

After that came another period of silence, broken only by the sounds of the two men undressing.  To keep himself awake Dick commenced to walk up and down the long hallway again.

“I guess I’ll call Tom,” he thought, at last, after more than an hour had passed.  “I’ve got to get some sleep, or I won’t be worth anything in the morning.  And if I am to call on Messrs. Pelter, Japson & Company I want to have my wits about me.”

He stepped around the corner of the hallway, in the direction where his own room was located.  He did not know that a man with eager eyes was watching him,—­ a man who stood on a chair in one of the rooms, peering through the transom light of the door.

“Gone at last—­ I was afraid he would stay here all night!” muttered the man.  “Now is my chance to get away.  I didn’t think they’d get here to-night.  I should have gotten that key made sooner.”  And opening the door noiselessly, he came out into the hallway.  He wore a thin overcoat and a slouch hat, and a heavy beard covered his face.