“Shake, Tom, old boy; ‘tip us your flipper,’ as Captain Andrews would say. The Wondership is ready for her final try-out.”

“Finished.” Tom Jesson drew a long sigh, then he wrung his cousin’s hand with energy enough to have wrenched it loose.

Jack Chadwick flung down the “alligator” wrench with which he had been going over every nut and bolt, and capered about the lofty, bare-raftered shed. Tom’s round face beamed, mirroring the other’s high good humor.

“And the try-out’s going to be a big success, Jack,” he declared positively. “I can feel it in my bones,—like Jupe when his rheumatics are coming on. My! Jack, that pontoon idea was the biggest thing we’ve ever struck.”

“Wait till we’ve tried it out,” smiled Jack, less impetuously; “it may prove the biggest bump we’ve ever struck.”

“Well, I’m willing to risk it. When shall we make the trial trip?”

“No time like the present. There are a few finishing touches still to be seen to, but by this evening everything will be ready. Besides, night is the best time. We don’t want a crowd around. There has been enough curiosity in what we have been doing, already.”

“I should say so. Look at this Boston sheet, will you? A column of mystery for a cent!”

Tom drew from his pocket a copy of a Boston paper and indicated some staring head-lines.

“’A Mystery of The Night Skies!’” he declaimed vociferously, waving an arm. “Some class there, eh?”

“Quite enough,” chuckled Jack. “We didn’t think that our little spin the other night was going to cause such a stir-up, did we?”

“It was all the fault of those red and green lights you hung out,” protested Tom. “Can you blame a community for getting worked up at the spectacle of colored lights like those on a ship, skimming around above their heads at sixty miles an hour? Hullo!” he broke off, still scanning the paper. “Here’s a letter from one fellow who declares that what was seen was a comet.”

“A comet, eh? Well, that wouldn’t be such a bad name for the new Flying Road Racer,” mused Jack reflectively.

“Never heard of a comet that would swim,” retorted Tom.

“Well, we don’t know yet that the new Road Racer will perform the stunts we expect her to.”

“In which case, we are in for a cold, cold bath.”

“Cheer up, Tom,” laughed Jack. “Get busy now and finish up the pontoons with that aluminum paint. If the trial is set for this evening, we haven’t any too much time.”

Both boys fell to work again with feverish energy. The work of many weeks, carried on sometimes in high hope, sometimes in deep despondency, was before them in complete form, except for the final touches. Only the important experiment remained. Would the re-modelled Flying Road Racer do what the boys expected of her? If the answer to that question was in the affirmative, they knew that they had invented and carried to perfection the greatest craft of its kind hitherto known. The new craft would indeed merit her name of Wondership if she did what the boys confidently expected of her.

And what was this Wondership that had for weeks occupied every minute of the Boy Inventors’ time, exclusive of their studies in the Technical College that both attended in Boston? Readers of former volumes of this series will recall the Flying Road Racer, the air and land ship that had carried the boys and their friends faithfully so many miles, and in which they had encountered many stirring adventures. Well, the Wondership, as Jack in his enthusiasm had termed the craft, was nothing more nor less than the Flying Road Racer, altered almost beyond recognition.

The shed in which the changes had been carried out was located on a lonesome part of the seacoast not far from Nestorville, where the boys lived. But, remote as the spot was, it still was not far enough removed from human haunts to escape much speculation over what was going forward in the great, gaunt, unpainted shed among the sand-hills.

Inquisitive folks had watched wagons, laden with big crates and seemingly heavy boxes, making their way to the place at intervals; but so carefully had the shed been guarded and locked that nobody had as yet discovered the boys’ secret. Had anyone done so, it is certain that the two lads would have been besieged by curiosity seekers, for the craft on which they were working was the most ambitious thing that they had undertaken. The Wondership was nothing more nor less than an invention capable of travel by land, air and water. On land it rolled along on wheels, above the earth it depended on a large, gas-filled bag for buoyancy, while on the water (and this was the feature still untested), the boys hoped to make it float like a boat by means of pontoons.

Of course, the idea of pontoons as applied to aerial craft was by no means a novelty. Glen Curtiss, pioneer in this field, already had a fleet of successful hydro-aeroplanes, and many other inventors were laboring along these lines. It was in the application of the idea that the boys had radically departed from anything hitherto known. At the risk of being tedious we must now describe the Wondership at some length, in order that what is to follow of her marvelous adventures may be clear.

Readers of former books relating the experience of the Boy Inventors know that the Flying Road Racer was a craft built like an immense automobile with a semi-cylindrical body. It seated six persons, and at a pinch could accommodate more. The lower part of the cylinder was a big tank in which gas was generated from a concentrated powder which, upon being mixed with water, formed a vapor of extraordinary buoyancy. In the upper part were padded seats, storage chambers for food and supplies, and a machinery chamber housed under a hood.

Above this auto-like structure rose a framework of vanadium and aluminum alloy, on which was folded, when not in use, the gas-bag which lifted the Flying Road Racer from the earth when it was desired to fly. Pumps filled the bag with gas, or withdrew it, as was desired. Provision allowing for the expansion and contraction of the bag had also been made, as was fully described in another volume.

What the boys had done was this: They had extended the semi-cylindrical formation till they had formed a full cylinder of light but strong metal. Roughly, the Flying Road Racer now resembled a huge, gleaming white cigar on wheels. Along her sides stretched hollow aluminum planes, or wings.

In the air these took the place of the former planes used in ascending or descending. On the water it was hoped that they would act as hydroplanes, buoying up the craft. But for buoyancy they did not depend on these hydroplanes, or pontoons, alone. The body of the Flying Road Racer was, by a singular stroke of inventive ingenuity, made to be in itself a buoyant craft.

When running along the road, or while flying, the top of the cylindrical body could be opened for air and observation. On a calm sea or lake the boys believed also that the craft, with the aid of the hydroplanes, would float, just like a boat. The hydroplanes at the side would, of course, correct a tendency to roll over, which an unsupported cylindrical body would naturally have. But in case of rough water, during which they might, in the course of the long flights they meant to take, be compelled to descend, the waves would be apt to break over the craft and swamp it.

To provide against such an emergency the ingenuity of the boys had been called into full play. It took many sleepless nights and days of anxious thought to solve the problem. But they believed that they had found a solution. The open space on the top of the cylinder was provided with metal doors which could be closed and screwed down, forming a water-tight compartment. Thus, the Flying Road Racer would, in a rough sea, be a water-tight cylinder, practically unsinkable unless the light metal hull was punctured.

The next problem had been a difficult one likewise. The question of how to ventilate an air-tight and water-tight cylinder was a vexing one. It was Jack who hit upon a plan. Like most big ideas it was simple, and was suggested to him by a recollection of the periscope tube on the submarine Peacemaker, which, as told in “The Boy Inventors and the Diving Torpedo Boat,” they had helped to construct. Jack’s solution, then, was this: A collapsible twin tube was made which when extended fully would reach upward, above the air-tight cylinder, to a height of twenty-five feet. At the bottom of this tube, and inside the cylinder, was a chamber containing two tiny fans. One of these fans, driven by storage batteries, sucked in fresh air from the top of the tube; the other drew out the foul fumes and sent them up the other channel of the extension pipe.

The Wondership was driven in the air and on land and water by the same power, the gas from the storage chamber which formed the lower section of the cylinder. But to fit her for her new work extra powerful engines had been installed, and a propeller of different pattern added. The propeller-shaft was connected to the motor through a water-tight stuffing box, as on a motor boat. The rudder lines, too, led through water-tight connections to the steering wheel. The aerial rudder, being of light metal like the propeller, was capable of use both in the air and water. In place of the old driving mechanism, too, the boys had simplified the Flying Road Racer by their new form of propeller. This did away with the cumbrous connections and clutches to the rear axle. The new form of propeller drew the Wondership along the roads almost as swiftly as it pulled her through the air.

As for the boys themselves, as readers of earlier volumes of this series know, they both lived at High Towers, the estate of Jack’s father, near Nestorville. Jack’s father was an inventor of note, and in our first story, “The Boy Inventors’ Wireless Triumph,” it was described how the boys aided him in many stirring adventures in Yucatan and in the discovery of Tom Jesson’s long missing father, an explorer and naturalist. Since that time Mr. Jesson had made his home with his brother-in-law who, like himself, was a widower. The next volume detailed how Jack and Tom helped an inventor in trouble, and how, after many perils and difficulties, a wonderful vanishing gun was at length brought to perfection in spite of the machinations of a gang of rascals. This volume was called “The Boy Inventors’ Vanishing Gun.”

The third volume has already been referred to. It told how the boys had many exciting times under the ocean and on the surface. The Peacemaker was a wonderful craft and proved of material aid to some Americans beleaguered by blood-thirsty negro revolutionists in Cuba. Through the experiences related in this book both the boys increased their mechanical ability and learned self-reliance and manliness in many a hard test of both those sterling qualities. Had this not been so, it is doubtful if they would ever have had the grit to bring to a triumphant conclusion the construction of the Wondership, beset as their way was oftentimes by apparently insurmountable difficulties. But now, as we know, the Wondership lay finished before them. Already they had tested her in flight to ascertain how she bore the added weight. It was this trial, on which she carried side lights, like a ship, that had caused the flurry in the city papers. It had been a complete success, and only the trial by water remained.

Although Mr. Chadwick and Mr. Jesson knew that the boys were engaged on a supreme task, neither had interfered or asked questions. Jack’s father believed in letting his son solve his own problems. He knew that if occasion arose his advice would be called for. But the boys meant to fight out their battle alone. Even the test to take place that evening was to be unwitnessed, or so they hoped. Not till all was an assured success did they intend to invite their parents to inspect their work.

As the term at the Technical College was over, both boys had full time to devote to their work. All day they labored with paint brush and wrench, testing and finishing. They gave themselves little time for lunch, eating with one hand and working with the other. So engrossed were they on their tasks that they did not notice that the brightness of the day outside was being dimmed rapidly. A spring storm was rolling up from seaward.

Neither did they know that their work was going forward with attention other than their own concentrated upon it. The unseen observer had alighted from a car at its terminal some miles away and tramped across the sand dunes toward the big shed. Keeping warily out of sight he made his way up to the structure and, boring a hole in the planking, watched with burning interest all that was going on within. He was an odd-looking figure, dressed in a loud checked suit and sporting a gaudy necktie and a hat cocked to one side. But his youthful face bore an inquiring, good-humored expression that belied his aggressive way of dressing. Over one shoulder was slung a camera. As he watched the boys through the small hole he had bored with a gimlet that he carried in his pocket, the unseen observer muttered strangely to himself.

“By the double-jointed hoorah of the Sahara Desert!” he exclaimed from time to time. “Dick, my boy, you’ve struck it! Instead of being fired for incompetency, you’ll be the biggest reporter in Boston to-morrow. You’ve run the Mystery of the Skies to its roost,—by the long-legged Llama of Thibet, you have!”

All day he watched, his joints stiff and aching from holding the one position, but he never budged. It was growing toward dusk before he observed the change in the weather that had come with startling suddenness. The sea, calm before, was now roaring angrily on the beach beyond the dunes. The sky was covered with scurrying clouds. The wind moaned ominously.

The unseen watcher made a grimace.

“In for a wetting and three miles to that car,” he muttered, “but by the crooked cantelope of Cambodia, it’s worth it! Hullo! What’s that?”

From seaward there had come the heavy boom of a gun. About four miles off shore, dangerously close for that coast, there lay a white, yacht-like craft. Clearly she had fired the gun. Now she ran up some sort of signal.

“By the scampering snakes of Senegambia, there’s another story!” gasped the watcher. “I’ll be made a managing editor at least, by the time I get through.”