for a few moments after their exclamations of surprise tom and mr. damon did not know what else to say. they stared about in amazement, hardly able to believe that the shed could be empty. they had expected to see some form of aeroplane in it, and tom was almost sure his eyes would meet a reproduction of his humming-bird, made from the stolen plans.

"can it be possible there's nothing here?" went on tom, after a long pause. he could not seem to believe it.

"evidently not," answered mr. damon, as he advanced toward the center of the big building and flashed the light on all sides. "you can see for yourself."

"or, rather, you can't see," spoke the youth. "it isn't here, that's sure. you can't stick an aeroplane, even as small a one as my humming bird, in a corner. no; it isn't here."

"well, we'll have to look further," went on mr. damon. "i think—"

but a sudden noise near the big main doors of the shed interrupted him.

"come on!" exclaimed tom in a whisper. "some one's coming! they may see us! let's get out!"

mr. damon released the pressure on the spring switch, and the light went out. after waiting a moment to let their eyes become accustomed to the darkness, he and tom stole to the door by which they had entered. as they swung it cautiously open they again heard the noise near the main portals by which andy had formerly taken in and out the anthony, as he had named the aeroplane in which he and his father went to alaska, where, like tom's craft, it was wrecked.

"some one is coming in!" whispered tom.

hardly had he spoken when a light shone in the direction of the sound. the illumination came from a big lantern of the ordinary kind, carried by some one who had just entered the shed.

"can you see who it is?" whispered mr. damon, peering eagerly forward; too eagerly, for his foot struck against the wooden side wall with a loud bang.

"who's there?" suddenly demanded the person carrying the lantern.

he raised it high above his head, in order to cast the gleams into all the distant corners. as he did so a ray of light fell upon his face. "andy foger!" gasped tom in a hoarse whisper.

andy must have heard, for he ran forward just as tom and mr. damon slipped out.

"hold on! who are you?" came in the unmistakable tones of the red-haired bully.

"i don't think we're going to tell," chuckled tom softly, as he and his friend sped off into the darkness. they were not followed, and as they looked back they could see a light bobbing about in the shed.

"he's looking for us!" exclaimed mr. damon with an inward laugh. "bless my watch chain! but it's a good thing we got in ahead of him. are you sure it was andy himself?"

"sure! i'd know his face anywhere. but i can't understand it. where has he been? what is he doing? where is he building his aeroplane? i thought he was out of town."

"he may have come back to-night," said mr. damon. "that's the only one of your questions i can answer. we'll have to wait about the rest, i'm sure he wasn't around the house to-day, though, for i was working at weeding the flower beds, in my disguise as a tramp, and if he was home i'd have seen him. he must have just come back, and he went out to his shed to get something. well, we did the best we could."

"indeed we did," agreed tom, "and i'm ever so much obliged to you, mr. damon."

"and we'll try again, when we get more clues. bless my shoelaces! but it's a relief to be able to talk as you like."

and forthwith the eccentric man began to call down so many blessings on himself and on his belongings, no less than on his friends, that tom laughingly warned him that he had better save some for another time.

the two reached home safely, removed their "disguises," and told mr. swift of the result of their trip. he agreed with them that there was a mystery about andy's aeroplane which was yet to be solved.

but tom was glad to find that, at any rate, the craft was not being made in shopton, and during the next two weeks he devoted all his time to finishing his own machine. mr. jackson was a valuable assistant, and mr. damon gave what aid he could.

"well, i think i'll be ready for a trial flight in another week," said tom one day, as he stepped back to get a view of the almost completed humming-bird.

"shall you want a passenger?" asked mr. damon.

"yes, i wish you would take a chance with me. i could use a bag of sand, not that i mean you are to be compared to that," added tom quickly, "but i'd rather have a real person, in order to test the balancing apparatus. yes, we'll make a trial trip together."

in the following few days tom went carefully over the aeroplane, making some slight changes, strengthening it here and there, and testing the motor thoroughly. it seemed to work perfectly.

at length the day of the trial came, and the humming-bird was wheeled out of the shed. in spite of the fact that it was practically finished, there yet remained much to do on it. it was not painted or decorated, and looked rather crude. but what tom wanted to know was how it would fly, what control he had over it, what speed it could make, and how it balanced. for it was, at best, very frail, and the least change in equilibrium might be fatal.

before taking his place in the operator's seat tom started the motor, and by means of a spring balance tested the thrust of the propellers. it was satisfactory, though he knew that when the engine had been run for some time, and had warmed up, it would do much better.

"all ready, i guess, mr. damon!" he called, and the odd gentleman took his place. tom got up into his own seat, in front of several wheels and levers by which he operated the craft.

"start the propeller!" he requested of mr. jackson, and soon the motor was spitting fire, while the big, fan-like blades were whirring around like wings of light. the engineer and eradicate were holding back the humming-bird.

"let her go!" cried tom as he turned on more gasoline and further advanced the spark of the motor. the roar increased, the propeller looked like a solid circle of wood, and the trim little monoplane moved slowly across the rising ground, increasing its speed every second, until, like some graceful bird, it suddenly rose in the air as tom tilted the wing tips, and soared splendidly aloft!