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
respo