"she seems to work," observed tom, looking from where he was stationed near some electrical switches, toward mr. sharp.

"of course she does," replied the aeronaut. "i knew it would, but i wasn't so sure that it would scoot along in this fashion. we're making pretty good speed, but we'll do better when the motor gets to running smoother."

"how high up are we?" asked tom.

the balloonist glanced at several gauges near the steering wheel.

"a little short of three thousand feet," he answered. "do you want to go higher?"

"no—no—i—i guess not," was tom's answer. he halted over the works, and his breath came in gasps.

"don't get alarmed," called mr. sharp quickly, noting that his companion was in distress because of the high altitude. "that always happens to persons who go into a thin air for the first time; just as if you had climbed a high mountain. breathe as slowly as you can, and swallow frequently. that will relieve the pressure on your ear drums. i'll send the ship lower."

tom did as he was advised, and the aeronaut, deflecting the rudder, sent the red cloud on a downward slant. tom at once felt relieved, both because the action of swallowing equalized the pressure on the ear drums, and because the airship was soon in a more dense atmosphere, more like that of the earth.

"how are you now?" asked the man of the lad, as the craft was again on an even keel.

"all right," replied tom, briskly. "i didn't know what ailed me at first."

"i was troubled the same way when i first went up in a balloon," commented mr. sharp. "we'll run along for a few miles, at an elevation of about five hundred feet, and then we'll go to within a hundred feet of the earth, and see how the red cloud behaves under different conditions. take a look below and see what you think of it."

tom looked low, through one of several plate glass windows in the floor of the car. he gave a gasp of astonishment.

"why! we're right over lake carlopa!" he gasped.

"of course," admitted mr. sharp with a laugh. "and i'm glad to say that we're better off than when i was last in the air over this same body of water," and he could scarcely repress a shudder as he thought of his perilous position in the blazing balloon, as related in detail in "tom swift and his motor-boat."

the lake was spread out below the navigators of the air like some mirror of silver in a setting of green fields. tom could see a winding river, that flowed into the lake, and he noted towns, villages, and even distant cities, interspersed here and there with broad farms or patches of woodlands, like a bird's-eye view of a stretch of country.

"this is great!" he exclaimed, with enthusiasm. "i wouldn't miss this for the world!"

"oh, you haven't begun to see things yet," replied mr. sharp. "wait until we take a long trip, which we'll do soon, as this ship is behaving much better than i dared to hope. well, we're five hundred feet high now, and i'll run along at that elevation for a while."

objects on the earth became more distinct now, and tom could observe excited throngs running along and pointing upward. they were several miles from shopton, and the machinery was running smoothly; the motor, with its many cylinders purring like a big cat.

"we could have lunch, if we'd brought along anything to eat," observed tom.

"yes," assented his companion. "but i think we'll go back now. your father may be anxious. just come here, tom, and i'll show you how to steer. i'm going down a short distance."

he depressed the rudder, and the red cloud shot earthward. then, as the airship was turned about, the young inventor was allowed to try his hand at managing it. he said, afterward, that it was like guiding a fleecy cloud.

"point her straight for shopton," counseled mr. sharp, when he had explained the various wheels and levers to the lad.

"straight she is," answered the lad, imitating a sailor's reply. "oh, but this is great! it beats even my motor-boat!"

"it goes considerably faster, at all events," remarked mr. sharp. "keep her steady now, while i take a look at the engine. i want to be sure it doesn't run hot."

he went aft, where all the machinery in the car was located, and tom was left alone in the small pilot house. he felt a thrill as he looked down at the earth beneath him, and saw the crowds of wonder-gazers pointing at the great, red airship flying high over their heads. rapidly the open fields slipped along, giving place to a large city.

"rocksmond," murmured tom, as he noted it. "we're about fifty miles from home, but we'll soon be back in the shed at this rate. we certainly are slipping along. a hundred and fifty feet elevation," he went on, as he looked at a gauge. "i wonder if i'll ever get used to going several miles up in the air?"

he shifted the rudder a bit, to go to the left. the red cloud obeyed promptly, but, the next instant something snapped. tom, with a startled air, looked around. he could see nothing wrong, but a moment later, the airship dipped suddenly toward the earth. then it seemed to increase its forward speed, and, a few seconds later, was rushing straight at a tall, ornamental tower that rose from one corner of a large building.

"mr. sharp! mr. sharp!" cried the lad. "something has happened! we're heading for that tower!"

"steer to one side!" called the balloonist.

tom tried, but found that the helm had become jammed. the horizontal rudder would not work, and the craft was rushing nearer and nearer, every minute, to the pile of brick and mortar.

"we're going to have a collision!" shouted tom. "better shut off the power!"

the two propellers were whirling around so swiftly that they looked like blurs of light. mr. sharp came rushing forward, and tom relinquished the steering wheel to him. in vain did the aeronaut try to change the course of the airship. then, with a shout to tom to disconnect the electric switch, the man turned off the power from the motor.

but it was too late. straight at the tower rushed the red cloud, and, a moment later had hit it a glancing blow, smashing the forward propeller, and breaking off both blades. the nose of the aluminum gas container knocked off a few bricks from the tower, and then, the ship losing way, slowly settled to the flat roof of the building.

"we're smashed!" cried tom, with something like despair in his voice.

"that's nothing! don't worry! it might be worse! not the first time i've had an accident. it's only one propeller, and i can easily make another," said mr. sharp, in his quick, jerky sentences. he had allowed some of the gas to escape from the container, making the ship less buoyant, so that it remained on the roof.

the aeronaut and tom looked from the windows of the car, to note if any further damage had been done. they were just congratulating themselves that the rudder marked the extent, when, from a scuttle in the roof there came a procession of young ladies, led by an elderly matron, wearing spectacles and having a very determined, bristling air.

"well, i must say, this is a very unceremonious proceeding!" exclaimed the spectacled woman. "pray, gentlemen, to what are we indebted for this honor?"

"it was an accident, ma'am," replied mr. sharp, removing his hat, and bowing. "a mere accident!"

"humph! i suppose it was an accident that the tower of this building was damaged, if not absolutely loosened at the foundations. you will have to pay the damages!" then turning, and seeing about two score of young ladies behind her on the flat roof, each young lady eying with astonishment, not unmixed with admiration, the airship, the elderly one added: "pupils! to your rooms at once! how dare you leave without permission?"

"oh, miss perkman!" exclaimed a voice, at the sound of which tom started. "mayn't we see the airship? it will be useful in our natural philosophy study!"

tom looked at the young lady who had spoken. "mary nestor!" he exclaimed.

"tom—i mean mr. swift!" she rejoined. "how in the world did you get here?"

"i was going to ask you the same question," retorted the lad. "we flew here."

"young ladies! silence!" cried miss perkman, who was evidently the principal of the school. "the idea of any one of you daring to speak to these—these persons—without my permission, and without an introduction! i shall make them pay heavily for damaging my seminary," she added, as she strode toward mr. sharp, who, by this time, was out of the car. "to your rooms at once!" miss perkman ordered again, but not a young lady moved. the airship was too much of an attraction for them.