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 a 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."
