Any one who has taken a flight in an aeroplane or gone up in a balloon, will know exactly how Mary Nestor felt on this, her first sky ride of any distance. For a moment, as she looked over the side of the machine, she had a distinct impression, not that she was going up, but that some one had pulled the earth down from beneath her and, at the same time, given her a shove off into space. Such is the first sensation of going aloft. Then the rush of air all about her, the slightly swaying motion of the craft, and the vibration caused by the motor took her attention. But the sensation of the earth dropping away from beneath her remained with Mary for some time.

This sensation is much greater in a balloon than in an aeroplane, for a balloon, unless there is a strong wind blowing, goes straight up, while an aeroplane ascends on a long slant, and always into the teeth of the wind, to take advantage of its lifting power on the underside of the planes. The reason for this sensation--that of the earth's dropping down, instead of one's feeling, what really happens, that one is ascending--is because there are no objects by which comparison can be made. If one starts off on the earth's surface at slow, or at great speed, one passes stationary objects--houses, posts, trees, and the like-- and judges the speed by the rapidity with which these are left behind.

Going up is unlike this. There is nothing to pass. One simply cleaves the air, and only as it rushes past can one be sure of movement. And as the air is void of color and form, there is no sensation of passing anything.

So Mary Nestor, as she shot into the air with Tom Swift, had a sensation as though the earth were dropping from beneath her. For a moment she felt as though she were in some vast void--floating in space--and she had a great fear. Then she calmed herself. She looked at Tom sitting in front of her. Of course, all she could see was his back, but it looked to be a very sturdy back, indeed, and he sat there in the aircraft as calmly as though in a chair on the ground. Then Mary took courage, and ceased to grasp the sides of the cockpit with a grip that stiffened all her muscles. She was beginning to "find herself."

On and on, and up and up, went Mary and Tom, in this the girl's first big sky ride. The earth below seemed farther and farther away. The wide, green fields became little emerald squares, and the houses like those in a toy Noah's ark.

Down below, Mr. Wakefield Damon, who had hurried over from his home in Waterfield to see Tom Swift, gazed aloft at the fast disappearing aeroplane and its passengers.

"Bless my coal bin!" cried the eccentric man, "but Tom is in a hurry this morning. Too bad he couldn't have stopped and spoken to me. It might have been greatly to his advantage. But I suppose I shall have to wait."

"You want to see Master?" asked a voice behind Mr. Damon, and, turning, he beheld a veritable giant.

"Yes, Koku, I did," Mr. Damon answered, and he did not appear at all surprised at the sight of the towering form beside him. "I wanted to see Tom most particularly. But I shall have to wait. I'll go in and talk to Mr. Swift."

"Yaas, an' I go talk to Radicate," said the giant. "Him diggin' up ground where Master told me to make garden. Radicate not strong enough for dat!"

"Huh! there's trouble as soon as those two get to disputing," mused Mr. Damon, as he went toward the house.

Meanwhile, Mary was beginning to enjoy herself. The sensation of moving rapidly through the air in a machine as skillfully guided as was the one piloted by Tom Swift was delightful. Up and up they went, and then suddenly Mary felt a lurch, and the plane, which was now about a thousand feet high, seemed to slip to one side.

Mary screamed, and began reaching for the buckle of the safety belt that fastened her to her seat. She saw that something unusual had occurred, for Tom was working frantically at the mechanism in front of him.

But, in spite of this, he seemed aware that Mary was in danger, not so much, perhaps, from what might happen to the machine, as what she might do in her terror.

"Oh! Oh!" cried the girl, and Tom heard her above the terrific noise of the motor, for she was speaking with her lips close to the tube that served as a sort of inter-communicating telephone for the craft. "Oh, we are falling! I'm going to jump!"

"Sit still! Sit still for your life!" cried Tom Swift. "I'll save you all right! Only sit still! Don't jump!"

Mary, her red cheeks white, sank back, and the young inventor redoubled his efforts at the controls and other mechanisms.

And that Tom was perfectly qualified to make a safe landing, even with engine trouble, Mary Nestor well knew. Those of you who have read the previous books of this series know it also, but, for the benefit of my new readers, I shall state that this was by no means Tom's first ride in an aeroplane.

He had operated and built gasoline engines ever since he was about sixteen years old. As related in the initial volume of this series, entitled, "Tom Swift and His Motorcycle," he became possessed of this machine after it had started to climb a tree with Mr. Damon on board. After that experience the eccentric man --blessing everything he could think of--had no liking for the speedy motorcycle and sold it to Tom at a low price.

That was the beginning of a friendship between the two, and also started Tom on his career as an inventor and a possessor of many gasoline craft. For he was not content with merely riding the repaired motorcycle. He made improvements on it.

Tom lived with his father in the town of Shopton, their home being looked after, since the death of Mrs. Swift, by Mrs. Baggert. Mr. Wakefield Damon lived in the neighboring town of Waterfield, and spent much time at Tom's home, often going on trips with him in various vehicles of the land, sea or air.

As related in the various volumes of this series, Tom was not content to remain on earth. He built a speedy motor boat, and then secured an airship, following that with a submarine. He also made an electric runabout that was the speediest car on the road. Sending wireless messages, having thrilling experiences among the diamond makers, journeying to the caves of ice, and making perilous trips in his sky racer took up part of the young inventor's time.

With his electric rifle he did some wonderful shooting, and in the "City of Gold" made some strange discoveries, part of the fortune he secured enabling him to build his sky racer. It was in a land of giants that Tom was made captive, but he succeeded in escaping, and brought two giants, of whom Koku was one, away with him.

Following this achievement Tom invented a wizard camera and a great searchlight, which, with his giant cannon, was purchased by the United States Government. Work on his photo-telephone and his aerial warship, the problem of digging a big tunnel, and then traveling to the land of wonders, kept Tom Swift very busy, and he had just completed a wonderful piece of work when the present story opens.

This last achievement was the perfecting of a machine to aid in the great World War and you will find the details set down in the volume which immediately precedes this. "Tom Swift and His War Tank," it is called, and in that is related how he not only invented a marvelous machine, but succeeded in keeping its secret from the plotters who tried to take it from him. In this Tom was helped by the inspiration of Mary Nestor, whom he hoped some day to marry, and by Ned Newton, a chum, who, though no inventor himself, could admire one.

Ned and Tom had been chums a long while, but Ned inclined more to financial and office matters than to machinery. At times he had managed affairs for Tom, and helped him finance projects. Ned was now an important bank official, and since the United States had entered the war had had charge of some Red Cross work, as well as Liberty Bond campaigns.

Somehow, as she sat there in the craft which seemed disabled, Mary Nestor could not help thinking of Tom's many activities, in some of which she had shared.

"Oh, if he falls now, and is killed!" she thought. "Oh, what will happen to us?"

"It's all right, Mary! Don't worry! It's all right!" cried Tom, through the speaking tube.

"What's that? I can't hear you very well !" she called back.

"No wonder, with the racket this motor is making," he answered. "Why can't something be done so you can talk in an aeroplane as well as in a balloon? That's an idea! If I could tell you what was the matter now you wouldn't be a bit frightened, for it isn't anything. But, as it is--"

"What are you saying, Tom? I can't hear you!" cried Mary, still much frightened.

"I say it's all right--don't get scared. And don't jump!" Tom shouted until his ears buzzed. "It's all nonsense--having a motor making so much noise one can't talk!" he went on, irritatedly.

A strange idea had come to the young inventor, but there was no time to think of it now. Mentally he registered a vow to take up this idea and work on it as soon as possible. But, just now, the aeroplane needed all his attention.

As he had told Mary, there was really nothing approaching any great danger. But it was rather an anxious moment. If Tom had been alone he would have thought little of it, but with Mary along he felt a double responsibility.

What had happened was that the craft had suddenly gone into an "air pocket" or partial vacuum, and there had been a sudden fall and a slide slip. In trying to stop this too quickly Tom had broken one of his controls, and he was busily engaged in putting an auxiliary one in place and trying to reassure Mary at the same time.

"But it's mighty hard trying to do that through a speaking tube with a motor making a noise like a boiler factory," mused the young inventor. Tom worked quickly and to good purpose. In a few moments, though to Mary they seemed like hours, the machine was again gliding along on a level keel, and Tom breathed more easily.

"And now for my great idea!" he told himself.

But it was some time before he could give his attention to that.