“Sparkler,” murmured Tom.  “I wonder what that can be?”

“That’s a slang word for diamonds,” suggested Ned.

“So it is.  In that case, dad, I think we have nothing to worry about.  Those fellows must be going to commit a diamond robbery or perhaps it has already taken place.”

The inventor seemed relieved at this theory of his son.  His face brightened and he said:  “If they are going to commit a robbery, Tom, we ought to notify the police.”

“But if they said that ‘Murdock,’ whoever he is, had the sparkler and was trying to get away with it without giving them their share, wouldn’t that indicate that the robbery had already taken place?” asked Ned.

“That’s so,” agreed Tom.  “But it won’t do any harm to tell the hotel detective that suspicious characters are around, no matter if the has been committed.  Then he can be on the lookout.  But I don’t think we have anything to worry about, dad.  Still, if you like, I’ll take a run down to the house to see that everything is all right, though I’m sure it will be found that we have nothing to be alarmed over.”

“Well, I will be more relieved if you do,” said the inventor, “However, suppose we have a good supper now and you boys can stay at the hotel to-night.  Then you and Ned can start off early in the morning.”

“All right,” agreed Tom, but there was a thoughtful look on his face and he appeared to be planning something that needed careful attention to details.

After supper that night Tom took his chum to one side and asked:  “Would you mind very much if you didn’t make the trip to Shopton with me?”

“No, Tom, of course not, if it will help you any.  Do you want me to stay here?”

“I think it will be a good plan.  I don’t like to leave dad alone if those scoundrels are around.  Of course he’s able to look after himself, but sometimes he gets absent minded from thinking too much about his inventions.”

“Of course I’ll stay here at the hotel.  This is just as good a vacation as I could wish.”

“Oh, I don’t mean all the while.  Just a day or so—­until I come back.  I may be here again by to-morrow night and find that my father is needlessly alarmed.  Then something may have happened at home and I would be delayed.  If I should be, I’d feel better to know that you were here.”

“Then I’ll stay, and if I see any of those men—­”

“You’d better steer clear of them,” advised Tom quickly.  “They are dangerous customers.”

“All right.  Then I’ll go over and give Miss Nestor lessons on how to run a motor-boat,” was the smiling response.  “I fancy, with what she and I know, we can make out pretty well.”

“Hold on there!” cried Tom gaily.  “No trespassing, you know.”

“Oh, I’ll just say I’m your agent,” promised Ned with a grin.  “You can’t object to that.”

“No, I s’pose not.  Well, do the best you can.  She is certainly a nice girl.”