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