again right now," Frank continued. "Hike for home, and get into some dry duds. I'll knock around for a spell, to try out a few more stunts I have in mind."

Truth to tell Frank was eager to get his new hydroplane away from those searching eyes of Mr. Marsh. They gave him a queer feeling, which of course he was quite unable to understand.

During the preceding summer, when the Bird boys were using the monoplane they had put together so successfully, it chanced that they had quite a serious adventure with a couple of thieves who had robbed a jewelry establishment, and were trying to get out of the country, where the roads were being closely watched by the police.

On this occasion one of the rascals chanced to be a man named Jules Garrone, who, over across the water had been something of an aeronaut and aviator. Conceiving the brilliant scheme that if the monoplane of the Bird boys could only be stolen he and his companion could easily elude their hunters, he had given Frank and Andy lots of trouble before finally falling into the net.

That was one reason why Frank felt rather cool toward strangers who manifested undue interest in his work. He was of an inventive turn of mind, and believed he had several new features connected with this hydroplane that as yet were, so far as he knew, novel to the science of aviation.

And those keen eyes of Mr. Marsh gave him an uneasy feeling.

"Your biplane seems to be built especially for two?" remarked that gentleman, as he watched Frank swing the machine around, with the help of the willing Larry and Elephant.

"Yes, sir," replied the boy, promptly. "Andy and myself always hunt together. We are called the Siamese Twins, because we won't be separated. Where one Bird boy is found you can make up your mind the other isn't very far away. Once on a time they got to calling me Smoke, and Andy, Fire; but we just wouldn't stand for that."

"But possibly your machine may not fly quite so well with only one to balance!"

Was that a broad hint that he would be only too glad of an invitation to occupy the seat left vacant by the departure of Andy? Frank suspected such a thing; and made a quick reply.

"Oh! we've got all that arranged to a dot, sir," he laughed. "I can change my seat, and still reach every lever easily. And as to balancing, the time has come when the aviator is going to be freed from all that anxiety. Give me a start, will you, fellows? It's easier rising from the water than on land, because no stumps or roots get in the way there. That's it. Good day, sir!"

There was a whirr of the powerful little Kinkaid engine, the lightest ever installed in an aeroplane, and immediately the new biplane started to take on speed. When, in the estimation of the one who handled the flier, it has attained sufficient momentum, the planes were elevated, and like a great bird it gracefully began to mount upward into space.

Larry was watching the two gentlemen in the car, who had been paying the closest attention to every little detail. He saw Mr. Marsh turn his head, and nod several times quickly to his companion.

"As neatly done as I ever saw it accomplished," the gentleman muttered, though the sharp ears of Larry Geohegan caught the suggestive words.

Then, after a few pleasant words to the two boys who had been fishing, the men in the touring car started off, heading toward town, and were speedily lost to sight in a cloud of dust.

"Let's take the short-cut, and bring up at the field where Frank and Andy do pretty much all of their practice, turning, and cutting figure-eights," suggested Larry, as though he had a purpose in saying this.

"Oh! I guess I'm still able to toddle that far," remarked Elephant who was compelled to work his short legs very fast when trying to keep alongside the taller Larry; and yet these two, so unlike in almost every way, had long been known as inseparables, ready to have an occasional little spat, yet just as quick to pour oil on the troubled waters again.

"There's Andy turning out," remarked Larry, after a while. "Perhaps, if you could only get going a little bit faster we might overtake him before he reaches home. I reckon he means to head for the shop in the field, because I know he always keeps a lot of old duds there."

"Sure thing," assented the dwarf, cheerfully, as he started on what was for him very like a run. "And it would be just like Andy to want to help when Frank comes along with the new biplane. Say, ain't she a dandy, though? Did you ever see such a neat contraption? Guess them gents thought we had some pretty smart fellows in Bloomsbury."

"That's just what I was thinking, Elephant," remarked Larry, "but here we are at the edge of the old field, and Andy just ahead. See that, he's aiming for the shop in the middle of the patch, where the hangar lies that holds their old monoplane. Perhaps you could buy that cheap now, Elephant. You know you always declared you meant to take up flying some day."

"Haven't given it up yet, either," returned the other, doggedly.

"Well, I advise you to think it over good and hard. Remember the fate of Darius Green. It needs a mighty active fellow to manage one of those tipsy, cranky machines. And if you ever should fall out I bet you there'd be an awful squash!" chuckled the tall boy.

"Let up on that, can't you?" expostulated Elephant. "I'm small, but I can get around as well as the next one. And when I get to sailing through the air, I expect to have wings. Then, if any accident comes along, it's me to flap my feathers, and drop like a thistle-down. In other words, Larry, I've got a parachute all arranged that will let me down easy; just like the fellow at the county fair, who drops from a hot air balloon."

"Hello! now what d'ye think of that?" claimed Larry, suddenly.

"What do you mean?" demanded the small boy. "You're the most mysterious fellow ever. Oh! I see now, by the way you stare over yonder. Yes, it's the same two gentlemen who admired the daring of the Bird boys a little while back. They must have found out where Andy lives, and have run out here from town to see what sort of a hangar they have."

"Yes, that's right, but I don't like it, I tell you," Larry went on, as he led the way over the fence that surrounded the field.

"What's that?" cried Elephant. "Sure you don't suspect these two fine looking gents might be another pair of crooks like the ones that tried to steal Frank's monoplane last summer, do you?"

"Oh! rats! You wouldn't understand if I did try to explain. There they go now, in a cloud of dust. Guess they saw us pointing at the car. Come along, slowpoke, and get up with Andy," and Larry linked his arm in that of his comrade, though he had to stoop considerably in order to make the connection.

"Why, hello, fellows!" exclaimed Andy, who now for the first time became aware of the fact that they had been trailing after him.

"Just dropped around to see if we could be of any use putting the new machine away," remarked Elephant, as if an apology were needed to account for their presence; but both boys had always been accounted special friends of Frank and Andy, and warmly greeted, though not taken into the secrets of the shop, where mystery reigned much of the time of late.

"And there's Frank coming right now!" declared Andy. "I guess he made up his mind he didn't care to put her through all her paces, with me away. We're sure proud of this new one, fellows. Why, she works like a clock, and minds her helm better than anything that ever answered to the call of the