hild could open a burglar-proof vault.

"It's no use," he moaned, and he sprawled at full length on the floor
of the tank, for there the air was purer. As he did so his fingers
touched something. He started as they closed around the handle of a big
monkey wrench. It was one he had brought into the place with him.
Imbued with new hope be struck a match and lighted his lantern, which
he had allowed to go out as it burned up too much of the oxygen. By the
gleam of it he looked to see if there were any bolts or nuts he could
loosen with the wrench, in order to slide the door back. It needed but
a glance to show him the futility of this.

"It's no go," he murmured, and he let the wrench fall to the floor.
There was a ringing, clanging sound, and as it smote his ears Tom
sprang up with an exclamation.

"That's the thing!" he cried. "I wonder I didn't think of it before. I
can signal for help by pounding on the sides of the tank with the
wrench. The blows will carry a good deal farther than my voice would."
Every one knows how far the noise of a boiler shop, with hammers
falling on steel plates, can be heard; much farther than can a human
voice.

Tom began a lusty tattoo on the metal sides of the tank. At first he
merely rattled out blow after blow, and then, as another thought came
to him, he adopted a certain plan. Some time previous, when he and Mr.
Sharp had planned their trip in the air, the two had adopted a code of
signals. As it was difficult in a high wind to shout from one end of
the airship to the other, the young inventor would sometimes pound on
the pipe which ran from the pilot house of the Red Cloud to the
engine-room. By a combination of numbers, simple messages could be
conveyed. The code included a call for help. Forty-seven was the
number, but there had never been any occasion to use it.

Tom remembered this now. At once he ceased his indiscriminate
hammering, and began to beat out regularly--one, two, three, four--then
a pause, and seven blows would be given. Over and over again he rang
out this number--forty seven--the call for help.

"If Mr. Sharp only comes back he will hear that, even in the house,"
thought poor Tom "Maybe Garret or Mrs. Baggert will hear it, too, but
they won't know what it means. They'll think I'm just working on the
submarine."

It seemed several hours to Tom that he pounded out that cry for aid,
but, as he afterward learned, it was only a little over an hour. Signal
after