"Well, Tom Swift, I don't believe you will make any mistake if you buy that diamond," said the jeweler to a young man who was inspecting a tray of pins, set with the sparkling stones. "It is of the first water, and without a flaw."

"It certainly seems so, Mr. Track. I don't know much about diamonds, and I'm depending on you. But this one looks to be all right."

"Is it for yourself, Tom?"

"Er--no--that is, not exactly," and Tom Swift, the young inventor of airships and submarines, blushed slightly.

"Ah, I see. It's for your housekeeper, Mrs. Baggert. Well, I think she would like a pin of this sort. True, it's rather expensive, but--"

"No, it isn't for Mrs. Baggert, Mr. Track," and Tom seemed a bit embarrassed.

"No? Well, then, Tom--of course it's none of my affair, except to sell you a good stone, But if this brooch is for a young lady, I can't recommend anything nicer. Do you think you will take this; or do you prefer to look at some others?"

"Oh, I think this will do, Mr. Track. I guess I'll take--"

Tom's Words were interrupted by a sudden action on the part of the jeweler. Mr. Track ran from behind the showcase and hastened toward the front door.

"Did you see him, Tom?" he cried. "I wonder which way he went?"

"Who?" asked the lad, following the shopkeeper.

"That man. He's been walking up and down in front of my place for the last ten minutes--ever since you've been in here, in fact, and I don't like his looks."

"What did he do?"

"Nothing much, except to stare in here as if he was sizing my place up."

"Sizing it up?"

"Yes. Getting the lay of the land, so he or some confederate could commit a robbery, maybe."

"A robbery? Do you think that man was a thief?"

"I don't know that he was, Tom, and yet a jeweler has to be always on the watch, and that isn't a joke, either, Tom Swift. Swindlers and thieves are always on the alert for a chance to rob a jewelry store, and they work many games."

"I didn't notice any particular man looking in here," said Tom, who still held the diamond brooch in his hand.

"Well I did," went on the jeweler. "I happened to glance out of the window when you were looking at the pins, and I saw his eyes staring in here in a suspicious manner. He may have a confederate with him, and, when you're gone, one may come in, and pretend to want to look at some diamonds. Then, when I'm showing him some, the other man will enter, engage my attention, and the first man will slip out with a diamond ring or pin. It's often done."

"You seem to have it all worked out, Mr. Track," observed the lad, with a smile. "How do you know but what I'm in with a gang of thieves, and that I'm only pretending to want to buy a diamond pin?"

"Oh, I guess I haven't known you, Tom Swift, ever since you were big enough to toddle, not to be sure about what you're up to. But I certainly didn't like the looks of that man. However, let's forget about him. He seems to have gone down the street, and, after all, perhaps I was mistaken. Just wait until I show you a few more styles before you decide. The young lady may like one of these," and the jeweler went to another showcase and took out some more trays of brooches.

"What makes you think she's a young lady, Mr. Track?" asked the lad.

"Oh, it's easy guessing, Tom. We jewelers are good readers of character. I can size up a young fellow coming in here to buy an engagement or a wedding ring, as soon as he enters the door. I suppose you'll soon be in the market for one of those, Tom, if all the reports I hear about you are true--you and a certain Mary Nestor."

"I--er--I think I don't care for any of these pins," spoke Tom, quickly, with a blush. "I like the first lot best. I think I'll take the one I had in my hand when that man alarmed you. Ha! That's odd! What did I do with it?"

Tom looked about on the showcase, and glanced down on the floor. He had mislaid the brooch, but the jeweler, with a laugh, lifted it out of a tray a moment later.

"I saw you lay it down," he said. "We jewelers have to be on the watch. Here it is. I'll just put it in a box, and--"

With an exclamation, Mr. Track gave a hasty glance toward his big show window. Tom looked up, and saw a man's face peering in. At the sight of it, he, too, uttered a cry of surprise.

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.

"WhyÄwhyÄÄ" stammered Tom.

The man knocked again.

"Tom--Tom Swift!" he called. "Don't you know me?"

"Know you--you?" repeated the lad.

"Yes Ä don't you remember Earthquake Island--how we were nearly killed there--don't you remember Mr. Jenks?"

"Mr. Jenks?"

Tom was so startled that he could only repeat words after the strange man, who was talking to him from outside the glass door.

"Yes, Mr. Jenks," was the reply. "Mr. Barcoe Jenks, who makes diamonds. I saw you in the store about to buy a diamond--I wanted to tell you not to--I'll give you a better diamond than you can buy--I just arrived in this place--I must have a private talk with you--Come out--I'll share a wonderful secret with you."

A flood of memory came to Tom. He did recall the very strange man who walked around Earthquake Island--where Tom and some friends had been marooned recently--walked about with a pocketful of what he said were diamonds. Now Barcoe Jenks was here.

"I must see you privately, Tom Swift," went on Mr. Jenks, as he once more tapped on the glass. "Don't waste money buying diamonds, when you and I can make better ones. Where can I have a talk with you? I--" Mr. Jenks suddenly looked down the dimly- lighted street. "They're coming back!" he cried. "I don't want to be seen. I'll call at your house later to-night--be on the watch for me--until then--good-by!"

He waved his hand, and was gone in an instant. Tom stood staring at the glass door. He hardly knew whether to believe it or not--perhaps it was all a dream.

He pinched himself to make sure that he was awake. Very substantial flesh met his thumb and finger, and he felt the pain.

"I'm awake all right," he murmured. "But Barcoe Jenks here--and still talking that nonsense about his manufactured diamonds. I think he must be crazy. I wonder--"

Once more the lad's musing was interrupted. He heard a murmur of excited voices outside the store, on the street. Then the door of the jewelry shop was tried. Mr. Track's face was pressed against the glass.

"Open the door! Let me in, Tom!" he called. "I've caught the thief," and as the lad unlocked the portal he saw that the jeweler held by the arm a ragged lad. "Ah; you scoundrel! I've caught you!" cried the diamond merchant, shaking the small chap, while Tom looked on, more mystified than ever.