The next instant the man outside knocked on the glass, apparently with a piece of metal, making a sharp sound.  As soon as he heard it, the jeweler once more sprang from behind the showcase, and leaped for the door crying: 

“There’s the thief!  He’s trying to cut a hole through my show window and reach in and get something!  It’s an old trick.  I’ll get the police!  Tom, you stay here on guard!” and before the lad could utter a protest, the jeweler had opened the door, and was speeding down the street in the gathering darkness.

Tom stared about him in some bewilderment.  He was left alone in charge of a very valuable stock of jewelry, the owner of which was racing after a supposed thief, crying: 

“Police!  Help!  Thieves!  Stop him, somebody!”

“This is a queer go,” mused Tom.  “I wonder who that man was?  He looked like somebody I know, and yet I can’t seem to place his face.  I wonder if he was trying to rob the placer Maybe there’s another one—­a confederate—­around here.”

This thought rather alarmed Tom, so he went to the door, and looked up and down the street.  He could see no suspicious characters, but in the direction in which the jeweler was running there was a little throng of people, following Mr. Track after the man who had knocked on the window.

“I wish I was there, instead of here,” mused the lad.  “Still I can’t leave, or a thief might come in.  Perhaps that was the game, and one of the gang is hanging around, hoping the store will be deserted, so he can enter and take what he likes.”

Tom had read of such cases, and he at once resolved that he would not only remain in the jewelry shop, but that he would lock the door, which he at once proceeded to do.  Then he breathed easier.

The town of Shopton, in the outskirts of which Tom lived with his father, and where the scene above narrated took place, was none too well lighted at night, and the lad had his doubts about the jeweler catching the oddly-acting man, especially as the latter had a good start.

“But some one may head him off,” reasoned Tom.  “Though if they do catch him, I don’t see what they can prove against him.  Hello, here I am carrying this diamond pin around.  I might lose it.  Guess I’ll put it back on the tray.”

He replaced in the proper receptacle one of the pins he bad been examining when the excitement occurred.

“I wonder if Mary will like that?” he said, softly.  “I hope she does.  Perhaps it would be better if she could come here herself and pick out one—­”

Tom’s musing was suddenly interrupted by a sharp tattoo on the glass door of the jewelry shop.  With a start, he looked up, to see staring in on him the face of the man who had been there before—­the man of whom the jeweler was even then in chase.

“Why—­why——­” stammered Tom.

The man knocked again.

“Tom—­Tom Swift!” he called.  “Don’t you know me?”