Tom made up his mind to find out if the stones were really diamonds, before saying anything to his father about the chance of going to seek Phantom Mountain.  And the young inventor wished to get the opinion of some other jeweler than Mr. Track—­at least, at first.

“Though if this one proves to be a good gem, I’ll have Mr. Track set it in a brooch, and give it to Mary for her birthday,” decided the young inventor.  “Guess I’ll take a run over to Chester in the Butterfly, and see what one of the jewelers there has to say.”

In addition to his big airship, Red Cloud, Tom owned a small, swift monoplane, which he called Butterfly.  This had been damaged by Andy Foger just before Tom left on the trip that ended at Earthquake Island, but the monoplane had been repaired, and Andy had left town, not having returned since.

Telling his father that he was going off on a little business trip, which he often did in his aeroplane, Tom, with the aid of Mr. Jackson, the engineer, wheeled the Butterfly out of its shed.

Adjusting the mechanism, and seeing that it was in good shape, Tom took his place in one of the two seats, for the monoplane would carry two.  Mr. Jackson then spun the propellers, and, with a crackle and roar the motor started.  Over the ground ran the dainty, little aeroplane, until, having momentum enough, Tom tilted the wing planes and the machine sailed up into the air.

Rising about a thousand feet, and circling about several times to test the wind currents, Tom headed his craft toward Chester, a city about fifty miles from Shopton.  In his pocket, snugly tucked away, were the two stones Mr. Jenks had given him.

It was not long before Tom saw, looming up in the distance the church spires and towering factory chimneys of Chester, for his machine was a speedy one, and could make ninety miles an hour when driven.  But now a slower speed satisfied our hero.

“I’ll just drop down outside of the city,” he reasoned, “for too much of a crowd gathers when I land in the street.  Besides I might frighten horses, and then, too, it’s hard to get a good start from the street.  I’ll leave it in some barn until I want to go back.”

Tom sent his craft down, in order to pick out a safe place for a landing.  He was then over the suburbs of the city, and was following the line of a straight country road.

“Looks like a good place there,” he murmured.  “I’ll shut off the motor, and vol-plane down.”

Suiting the action to the word, Tom shut off his power.  The little craft dipped toward the ground, but the lad threw up the forward planes, and caught a current of air that sent him skimming along horizontally.

As he got nearer to the ground, he saw the figure of a lad riding a bicycle along the country highway.  Something about the figure struck Tom as being familiar, and he recognized the cyclist a moment later.

“It’s Andy Foger!” said Tom, in a whisper.  “I wondered where he had been keeping himself since he damaged the Butterfly.  Evidently he doesn’t dare venture back to Shopton.  Well, here’s where I give him a scare.”