while dr. hendrix was in his office, getting ready to make the thrilling trip through the air with tom, the young inventor spent a few minutes going over his monoplane. the wonderful little craft had made her first big flight in excellent time, though tom knew she could do better the farther she was flown. not a stay had started, not a guy wire was loose. the motor had not overheated, and every bearing was as cool as though it had not taken part in thousands of revolutions.

"oh, i can depend on you!" murmured tom, as he looked to see that the propeller was tight on the shaft. he gave the bearing a slight adjustment to make sure of it.

he was at this when the specialist reappeared. dr. hendrix, after his first show of excitement, when he had made his decision to accompany tom, had resumed his usual calm demeanor. once again he was the grave surgeon, with his mind on the case before him.

"well, is my auto ready?" he asked absentmindedly. then, as he saw the little aeroplane, and tom standing waiting beside it, he added: "oh, i forgot for the moment that i was to make a trip through the air, instead of in my car. well, mr. swift, are we all ready?"

"all ready," replied the young inventor. "we're going to make fast time, dr. hendrix. you'd better put this on," and tom extended a face protector.

"what's it for?" the physician looked curiously at it.

"to keep the air from cutting your cheeks and lips. we are going to travel a hundred miles an hour this trip."

"a hundred miles an hour!" dr. hendrix spoke as though he would like to back out.

"maybe more, if i can manage it," went on tom, calmly, as he proceeded to remove the bag of sand from the place where the surgeon was to sit. then he looked to the various equilibrium arrangements and the control levers. he was so cool about it, taking it all for granted, as if rising and flying through the air at a speed rivaling that of the fastest birds, was a matter of no moment, that dr. hendrix was impressed by the calm demeanor of the young inventor.

"very well," said the surgeon with a shrug of his shoulders, "i guess i'm game, tom swift."

the doctor took the seat tom pointed out to him, with his bag of instruments on his knees. he put on the face protector, and had, at the suggestion of our hero, donned a heavy coat.

"for it's cold in the upper regions," said tom.

several servants in the physician's household had gathered to see him depart in this novel fashion, and the chauffeur of the auto, in which the specialist usually made his calls, was also there.

"i'll give you a hand," said the chauffeur to the young inventor. "i was at an aviation meet once, and i know how it's done."

"good," exclaimed tom. "then you can hold the machine, and shove when i give the word."

tom started the propeller himself, and quickly jumped into his seat. the chauffeur held back the humming-bird until the young aviator had speeded up the motor.

"let go!" cried the youthful inventor, and the man gave the little craft a shove. across the rather uneven ground of the doctor's yard it ran, straight for a big iron barrier.

"look out! we'll be into the fence!" shouted the surgeon. "we'll be killed!" he seemed about to leap off.

"sit still!" cried tom, and at that instant he tilted the elevation planes, and the craft shot upward, going over the fence like a circus horse taking a seven-barred gate.

"oh!" exclaimed the physician in a curious voice. they were off on their trip to save the life of mr. swift.

what the sensations of the celebrated specialist were, tom never learned. if he was afraid, his fright quickly gave place to wonder, and the wonder soon changed to delight as the machine rose higher and higher, acquired more speed, and soared in the air over the country that spread out in all directions from kirkville.

"magnificent! magnificent!" murmured the doctor, and then tom knew that the surgeon was in the grip of the air, and was one of the "bird-men."

every moment the humming-bird increased her speed. they passed over the river near where men were working on the broken bridge. it was now no barrier to them. tom, noting the barograph, and seeing that they were twenty-two hundred feet high, decided to keep at about that distance from the earth.

"how fast are we going?" cried dr. hendrix, into the ear of the young inventor.

"just a little short of a hundred an hour!" tom shouted back. "we'll hit a hundred and five before long."

his prediction proved true, and when about forty miles from shopton that terrific speed had been attained. it seemed as if they were going to have a trip devoid of incident, and tom was congratulating himself on the quick time made, when he ran into a contrary strata of air. almost before he knew it the humming-bird gave a dangerous and sickening dive, and tilted at a terrifying angle.

"are we going to turn turtle?" cried the doctor.

"i—i hope not!" gasped tom. he could not understand why the equilibrium weights did not work, but he had no time then to investigate. quickly he warped the wing tips and brought the craft up on an even keel.

he gave a sigh of relief as the aeroplane was once more shooting forward, and he was not mistaken when he thought he heard dr. hendrix murmur a prayer of thankfulness. their escape had been a narrow one. tom's nerve, and the coolness of the physician, had alone saved them from a fall to death.

but now, as if ashamed of her prank, the humming-bird went along even better than before. tom was peering through the slight haze that hung over the earth, for a sight of shopton. at length the spires of the churches came into view.

"there it is," he called, pointing downward. "we'll land in two minutes more."

"no time to spare," murmured the doctor, who knew the serious nature of the aged inventor's illness. "how long did it take us?"

"fifty-one minutes," replied tom, glancing at a small clock in front of him. then he shut off the motor and volplaned to earth, to the no small astonishment of the surgeon. he made a perfect landing in the yard before the shed, leaped from his seat, and called:

"come, dr. hendrix!"

the surgeon followed him. dr. gladby and dr. kurtz came to the door of the house. on their faces were grave looks. they greeted the celebrated surgeon eagerly.

"well?" he asked quickly, and they knew what he meant.

"you are only just in time," said dr. gladby, softly, and tom, following the doctors into the house, wondered if his trip with the specialist had been in vain.