Tom Swift was busy with some apparatus on the Hawk. Ned heard the hum of an electric motor which was connected with the engine, and there soon sounded the crackle of the wireless.
"What are you doing? Signaling for help from those inside the tank?" asked Ned, for the big machine was fitted to receive and send messages of this sort.
"I'm trying something more desperate than that," Tom answered.
Again the wireless crackled, Tom working it with one hand while, with the other, he guided the aircraft. Ned looked downward with wondering eyes.
The tank was still plunging her way toward the steep bank of the river. If she tumbled down this, there would be little left of the expensive and complicated machinery inside.
"The rascals did their work well," mused Ned. "They've probably gotten all the secrets they want and now they're going to spoil all Tom's hard work. It's a shame! If only—"
Ned ceased his musing. Something was taking place down below that he could not explain. The tank seemed to be slackening her progress. More and more slowly she approached the edge of the cliff.
"Tom! Tom!" yelled Ned. "You must have waked some of them up inside and they've thrown the motors out of gear! Hurrah! She's stopping!"
"I believe she is!" yelled Tom. "Oh, if it only works!"
The tank was still moving, though more slowly. Still the crackle of the wireless was heard.