As Jack spoke, the island gave another trembling shake. It was only a slight one, but it warned them that, in all probability, there were to be more violent shocks succeeding it.

It was plain enough that their escape, if it was to be made at all, must be made quickly. Jack and Tom at once set about dismantling the wireless station and packing the apparatus.

The hastily extemporized life jacket balloons were hauled down and the wires coiled. When this had been done. Jack told everybody to take their seats in the car, on the top of which the dismantled gas bag had been folded by the captain and the two sailors, while Abner Jennings helped Jupe to pack up.

Jack took his seat last of all and started the engine going. It worked without a hitch, and the auto,—a flying machine no longer,—moved off across the sand, heavily laden as it was, without difficulty.

The rim about the submerged centre of the island was soon circumnavigated, and the beginning of the narrow neck of land reached. Then Jack fairly “let the car out.”

The newly formed isthmus was hard, and the car flew over it under the full power of its engines.

“Mighty good t’ing dere ain’t no speed laws in dis part ob de world,” grunted Jupe as they flew along.

The shore appeared to rush toward them, but if they had hoped to see any signs of human habitation as they drew close to it they were mistaken. Nothing but a mass of trees, backed by rising ground, appeared along the coast as far as the eye could reach in either direction.

As they sped along they heard behind them a sudden mighty uproar. Gazing back they saw the ocean heaving and boiling all about the island they had left, as if it had been a witches’ caldron. Great jets of water shot up, and the surface of the sea was flecked with foam and spume.

The sight fascinated every one of them but Jack, who had to be intent on his driving.

“The whole island is going!” shouted the Professor.

He was right.

With a sudden booming roar and upheaval of the ocean, the entire mass of land sank under the waves, which for a long time boiled and simmered above it. Just as the last vestige of the island vanished, leaving only the newly created peninsula projecting from the land, they reached the solid earth.

Their dash to the mainland had taken place only just in time. A little more delay, they realized with shudders, would have meant their total annihilation.

“I said the island would go,” cried Abner Jennings triumphantly. “I’ve ’em vanish like that in the South Seas.”

No one had any comment to make. The horror of what they had just witnessed struck them all dumb. The gratitude they felt to Divine Providence for their lucky rescue filled their hearts to overflowing, and left no room for speech.

The Flying Road Racer was stopped, and they silently gazed for a long time at the bubbling, heaving waters.

The sight was impressive, even if it did cause a shiver and inspire a feeling that bordered on fear.

After a while the Professor spoke. His tone was as solemn as his words.

“Boys,” he said, addressing his young friends, “we have just witnessed something that many scientists would give a great deal to behold.”

“Well, candidly,” said Tom, “I’ve seen enough of it.”

So had they all, in fact, and the Flying Road Racer was soon turned north, following a rough road that ran parallel with the sea-coast.

It was now late afternoon, and the shadows were lengthening apace. Before long the swift tropic night would overtake them. Although they had arrived at a determination to continue traveling north till they arrived at a large city, where a telegraph wire could be found, they did not care to risk advancing over the rough, half-formed road in the darkness, so a halt was made where a small stream of fresh water ran down to the sea, and they prepared to spend the night there.

It was somewhat chilly and a roaring fire was built around which they seated themselves after the evening meal. All were rather silent and abstracted, and there was no inclination for conversation. The Professor had brought out the silver casket and was examining some queer marks like hieroglyphics on its cover.

“I’m sure they have some sort of meaning,” he remarked to Mr. Jesson, “but it’s beyond me to make out what it can be. See if you can do any better.”

He handed the box to his brother-in-law to examine. But in the transfer it was fumbled, and before Mr. Jesson could save it the silver casket rolled toward the fire, only stopping when it was embedded in a mass of embers.

It was raked out with a stick by Mr. Jesson before it was damaged. He set it aside to cool before examining it, and in the meantime the boys took occasion to observe it more narrowly than they had yet found opportunity to do.

“Say, I thought that those knobs on the top were dull-colored!” exclaimed Jack Chadwick suddenly.

“Why, so they are!” rejoined Mr. Jesson. “Some sort of inferior stone, I guess. They——”

“But they are not dull! Look!”

Risking burning his fingers. Jack seized the still warm casket and held it toward his elders.

On the cover, embedded in the silver, flashed and winked in the firelight, three magnificent gems, red, blue, green!

“Let me look at that a minute. Jack,” exclaimed Professor Chadwick in sharp, excited tones.

He took the box from his son, and an instant later his head and Mr. Jesson’s were close together over the rifled silver casket.

“Well, gentlemen?” said Ned after a while.

“Well,” echoed Professor Chadwick, “we have made a most astounding discovery. These gems which Jack discovered,—for they are genuine, there’s not a doubt of it,—must have been covered with wax of some sort. The heat of the fire, when the box fell into it, melted this substance, and—well, here are three gems worth, conservatively, two hundred and fifty thousand dollars; probably a great deal more.”

The listeners looked at him in amazement.

“But what were the gems that Herrera took out of the casket, then?” demanded Jack, when he found his voice.

“Imitations, undoubtedly,” was the reply of Mr. Jesson. “The tribe that owned the genuine stones adopted this cunning means of concealing the real ones by coating them with wax of some sort. Then they placed inferior gems, or cunning imitations, within the box, trusting to the cupidity of any one who stole them not to investigate further.”

And so it proved afterward. The stones, which the strange and seemingly trivial accident had revealed, turned out to be as fine specimens of their respective kinds as there are in existence. They were appraised at six hundred and eighty thousand dollars, but cryptic carvings on the back of them made them of infinitely more value to science as specimens of the treasures of a vanished race.

Despite their keen excitement over the discovery that, after all, Herrera had not decamped with the precious stones, the adventurers slept soundly and peacefully that night.

When they awakened the daylight was sparkling on land and sea, and Jupe was filling the air with appetizing aromas proceeding from his cooking fire.

It was while they were in the midst of the morning meal that Jack sprang to his feet with a shout.

“The Sea King! the Sea King!” he cried, pointing seaward.

About half a mile off shore, steaming leisurely along, was a fine-looking white yacht that the Professor speedily pronounced to be, indeed, the Sea King.

“The wireless, Tom, as quick as you can,” called Jack, and the two lads at once set about sending their life-jacket balloons aloft.

This time the message that Jack sent out reached the persons it was intended for, and an hour later a boat came ashore and the castaways found themselves among their friends.

Repairs had been effected in record time on the yacht, and those in charge of her had determined not to wait longer at Lone Island, but proceed south at once. They were urged to this course, also, by news from Mexico that the revolutionists had triumphed, and that Diaz had abdicated.

We should like to chronicle more of the adventures of the Boy Inventors on this trip, but the exigencies of space forbid it. Suffice it to say then, that while the Professor, the rescued explorer and the rest, including Captain Andrews, voyaged to Lone Island and thence home on the Sea King, the boys drove the Flying Road Racer through Mexico, and reached home in that way by the overland route. They had many exciting times, but none so filled with peril and incident as their career on the gulf had been.

In due time the Vagrant was also recovered and sent home by the newly formed Madero government. Of Herrera, all trace was lost for a time. But ultimately he was heard from in Paris, whither, as had been prophesied, he had fled when the Diaz government fell. But he is not leading the life of a luxurious refugee there. Far from it. The gems he had stolen with the exercise of so much villainy and planning, proved to be, as Professor Chadwick had conjectured, mere cheap imitations worth very little except as specimens of Maya workmanship. Herrera, when last heard from, was acting as a head waiter in an humble Mexican restaurant in the Latin quarter of the French capital.

The genuine gems were sold to a New York millionaire, and when he dies will be seen in his private museum, which will then be opened to the public. The proceeds were shared, by the wishes of Professor Chadwick and Mr. Jesson, with the faithful crew of the Sea King, each, from Captain Andrews down, receiving a due portion. A handsome monument was also erected above the grave of poor Kettle, who fell in the battle with the Mayas.

Professor Chadwick did not fulfill the object of his cruise in finding a new form of biologic life; but he often says that he established something far more precious,—namely, the safety of his long-lost brother-in-law, Tom Jesson’s father.

One morning, not long after the household at High Towers had settled down to its ordinary routine, a telegram came for Jack. It contained astonishing things, things which were—though he didn’t guess it at the time,—to open up an entirely new field of invention for him and his chums, Tom Jesson and Ned Bangs.

The message stated,—but positively, we must keep all that for another telling. In our next volume we will relate further astonishing and stirring occurrences in the lives of our ingenious, progressive young friends. The title of the forthcoming book will be The Boy Inventors and the Vanishing Gun,—a tale which promises to be of extraordinary interest to every American boy, brimful and running over, as it will be, with experiment and achievement along new and significant lines.