tom lost no time in writing to mr. sharp. he wondered more and more at his own neglect in not before having asked the balloonist, when the latter was in shopton, where andy was building his aeroplane. but, as it developed later, mr. sharp did not know at that time.

while waiting for a reply to his letter, tom busied himself about his own craft, making several changes he had decided on. he also began to paint and decorate it, for he wanted to have the humming-bird present a neat appearance when she was officially entered in the great race.

miss nestor called on tom again, and mr. damon was a frequent visitor. he agreed to accompany tom to the aviation park when it was time for the race, and also to be a passenger in the ten-thousand-dollar contest.

"it must be perfectly wonderful to fly through the air," said miss nestor one day, when tom and mr. damon had the humming-bird out on the testing ground, trying the engine, which had been keyed up to a higher pitch of speed. "i consider it perfectly marvelous, and i can't imagine how it must seem to skim along that way."

"come and try it," urged tom suddenly. "there's not a bit of danger. really there isn't."

"oh! i'd never dare do it!" replied the girl, with a gasp. "that machine is too swift by name and swift by nature for me."

"why don't you take miss nestor on a grass-cutting flight, tom?" suggested mr. damon. "bless my lawn mower! but she wouldn't be frightened at that."

"grass cutting?" repeated the girl. "what in the world does that mean?"

"it means skimming along a few feet up in the air," answered the young inventor, who had now fully recovered from the effects of the blow given him by the midnight intruder. in spite of many inquiries, no clues to his identity had been obtained.

"how high do you go when you 'cut grass,' as you call it?" asked miss nestor, and tom thought he detected a note of eager curiosity in her voice.

"not high at all," he said. "in fact, sometimes i do cut off the tops of tall daisies. come, mary! won't you try that? i know you'll like it, and when you've been over the lawn a few times you'll be ready for a high flight. come! there's no danger."

"i—i almost believe i will," she said hesitatingly. "will you take me down when i want to come?"

"of course," said tom. "get in, and we'll start."

the humming-bird was all ready for a trial flight, and tom was glad of the chance to test it, especially with such a pretty passenger as was miss nestor.

"bless my shoelaces!" cried mr. damon. "i can see where i am going to be cut out, tom swift. i'll not get many more rides with you now that miss nestor is taking to aeroplaning, you young rascal!" and he playfully shook his finger at tom.

"oh, i don't expect to get enthusiastic over it," said miss nestor, who, now that she had taken her place in one of the small seats under the engine, appeared as if she would be glad of the chance to change her mind. but she did not.

"now, if you take me more than five feet up in the air, i'll never speak to you again, tom swift!" she exclaimed.

"five feet it shall be, unless you yourself ask to go higher," was the youth's reply, as he winked at mr. damon. well he knew the fascination of aeroplaning, and he was almost sure of what would happen. "you can take a tape measure along, and see for yourself," he added to his fair passenger. "the barograph will hardly register such a little height."

"well, it's as high as i want to go," said the girl. "oh!" with a scream, as tom started the propeller. "are we going?"

"in a moment," was his reply. he took his seat beside the girl. the motor was speeded up until it sounded like the roar of the ocean surf in a storm.

"let her go!" cried tom to mr. damon and mr. jackson, who were holding back the humming-bird. they gave her a slight shove to over-come the inertia, and the trim little craft darted across the ground at every increasing speed.

miss nestor caught her breath with a gasp, glanced at tom, and noted how cool he was, and then her frantic grip of the uprights slightly relaxed.

"we'll go up a little way in a minute!" shouted tom in her ear as they were speeding over the level ground.

he pulled a lever slightly, and the humming-bird rose a little in the air, but only for a short distance, not more than five feet, and tom held her there, though he had to run the engine at a greater speed than would have been the case had he been in the sustaining upper currents. it was as if the humming-bird resented being held so closely to the earth.

around in a big circle, back and forth went the craft, at no time being more than seven feet from the ground. tom glanced at miss nestor. her cheeks were unusually red, and there was a bright sparkle in her eyes.

"it's glorious!" she cried. "do you—do you think there's any danger in going higher? i believe i'd like to go up a bit."

"i knew it!" cried tom. "up we go!" and he pulled the wind-bending plane lever toward him. upward shot the craft, as if alive.

"oh!" gasped mary.

"sit still! it's all right!" commanded tom.

"it's glorious; glorious!" she cried. "i'm not a bit afraid now!"

"i knew you wouldn't be," declared the young inventor, who had calculated on the fascination which the motion through the air, untrammeled and free, always produces. "shall we go higher?"

"yes!" cried miss nestor, and she gazed fearlessly down at the earth, which was falling away from beneath their feet. she was in the grip of the air, and it was a new and wonderful sensation.

tom went up to a considerable distance, for, once a person loses his first fright, one hundred feet or one thousand feet elevation makes little difference to him. it was this way with miss nestor.

now, indeed, could tom demonstrate to her some of the fine points of navigation in the upper currents, and though he did no risky "stunts," he showed the girl what it means to do an ascending spiral, how to cut corners, how to twist around in the figure eight, and do other things. tom did not try for the great speed of which he knew his craft was capable, for he knew there was some risk with miss nestor aboard. but he did nearly everything else, and when he sent the humming-bird down he had made another convert and devotee to the royal sport of aeroplaning.

"oh! i never would have dared believe i could do it!" exclaimed the girl, as with flushed cheeks and dancing eyes she dismounted from the seat. "mamma and papa will never believe i did it!"

"bring them over, and i'll take them for a flight," said tom, with a laugh, as mary departed.

tom received an answer to his letter to mr. sharp that night.

"andy foger's entry blank states," wrote the balloonist, "that he is constructing his aeroplane in the village of hampton, which is about fifty miles from your place. if there is anything further i can do for you, tom, let me know. i will see you at the meet. hope you win the prize."

"in hampton, eh?" mused tom. "so that's where andy has been keeping himself all this while. his uncle lives there, and that's the reason for it. he wanted to keep it a secret from me, so he could use my stolen plans for his craft. but he shan't do it! i'll go to hampton!"

"and i'll go with you!" declared mr. damon, who was with tom when he got the note from the balloonist. "we'll get to the bottom of this mystery after a while, tom."

delaying a few days, to make the final changes in his aeroplane, tom and mr. damon departed for hampton one morning. they thought first of going in the butterfly, but as they wanted to keep their mission as secret as possible, they decided to go by train, and arrive in the town quietly and unostentatiously. they got to hampton late that afternoon.

"what's the first thing to be done?" asked mr. damon as they walked up from the station, where they were almost the only persons who alighted from the train.

"go to the hotel," decided tom. "there's only one, i was told, so there's not much choice."

hampton was a quiet little country town of about five thousand inhabitants, and tom soon learned the address of mr. bentley, andy's uncle, from the hotel clerk.

"what business is mr. bentley in?" asked tom, for he wanted to learn all he could without inquiring of persons who might question his motives.

"oh, he's retired," said the clerk. "he lives on the interest of his money. but of late he's been erecting some sort of a building on his back lot, like a big shed, and folks are sort of wondering what he's doing in it. keeps mighty secret about it. he's got a young fellow helping him."

"has he got red hair?" asked tom, while his heart beat strangely fast.

"who? mr. bentley? no. his hair's black."

"i mean the young fellow."

"oh! his? yes, his is red. he's a nephew, or some relation to mr. bentley. i did hear his name, but i've forgotten it. sandy, or andy, or some such name as that."

this was near enough for tom and mr. damon, and they did not want to risk asking any more questions. they turned away to go to their rooms, as the clerk was busy answering inquiries from some other guests. a little later, supper was served, and tom, having finished, whispered to mr. damon to join him upstairs as soon as he was through.

"what are you going to do?" asked the eccentric man.

"we're going out and have a look at this new shed by moonlight," decided tom. "i want to see what it's like, and, if possible, i want to get a peep inside. i'll soon be able to tell whether or not andy is using my stolen plans."

"all right. i'm with you. bless my bill of fare! but we seem to be doing a lot of mysterious work of late."

"yes," agreed tom. "but if you have to bless anything to-night, mr. damon, please whisper it. andy, or some of his friends, may be about the shed, and as soon as they hear one of your blessings they'll know who's coming."

"oh, i'll be careful," promised mr. damon.

"andy will find out, sooner or later, that we are in town," went on tom, "but we may be able to learn to-night what we want to know, and then we can tell how to act."

a little later, as if they were merely strolling about, mr. damon and tom headed for mr. bentley's place, which was on the outskirts of the town. there was a full moon, and the night was just right for the kind of observation tom wanted to make. there were few persons abroad, and the young inventor thought he would have no one spying on him.

they located the big house of andy's uncle without trouble. going down a side street, they had a glimpse of a shed, built of new boards, standing in the middle of a large lot. about the structure was a new, high wooden fence, but as tom and his friend passed along it they saw that a gate in it was open.

"i'm going in!" whispered tom.

"will it be safe?" asked mr. damon.

"i don't care whether it will be or not. i've got to know what andy is doing. come on! we'll take a chance!"

cautiously they entered the enclosure. the big shed was dark, and stood out conspicuously in the moonlight.

"there doesn't seem to be any one here," whispered tom. "i wonder if we could get a look in the window?"

"it's worth trying, anyhow," agreed mr. damon. "i'm with you, tom."

they drew nearer to the shed. suddenly tom stepped on a stick, which broke with a sharp report.

"bless my spectacles!" cried mr. damon, half aloud.

there was silence for a moment, and then a voice cried out:

"who's there? hold on! don't come any farther! it's dangerous!"

tom and mr. damon stood still, and from behind the shed stepped andy foger and a man.

"oh! it's you, is it, tom swift?" exclaimed the red-haired bully. "i thought you'd come sneaking around. come on, jake! we'll make them wish they'd stayed home!" and andy made a rush for tom.