He broke off with a guttural oath in German, as a low droning overhead came to his ears. Nelson heard it, too, and raised his night glasses to sweep the sky for the source of the sound.
Suddenly he clutched Brandt by the arm and handed the glasses to him. "There! Look!"
Brandt took the binoculars and focused them to suit his own vision.
"It is some sort of airplane," he said a few minutes later, returning the glasses to Nelson. "A queer-looking one. I can't quite make it out!"
Nelson took the glasses and refocused them. There was something queer about the gliding motion of the aircraft, and then as it came closer, he could make out that it had a second propeller overhead.
"It's a helicopter!" he exclaimed softly. "But it hardly makes any sound at all—a noiseless helicopter. I bet it's Diesel-engined!"
"Ja?" There was surprise in Herr Kommander Brandt's tone.
"And I'll bet," Nelson went on, his eyes suddenly flashing and his voice quivering with excitement, "that Curtis has sent it after us. He always advocated the freer use of helicopters, especially the Diesel-engined type. It wouldn't surprise me if he's in it!"
"Ve can blast him out of the air mit Androka's ray!" Brandt said. "Like ve blasted those bombers!"
"Maybe!" Nelson's tone held a touch of doubt. "We can try!" He used his telephone to call Androka. "Get busy, Androka!" he said into the mouthpiece. "Blast it with your plane-destroyer! Come up right away!"