"The same; only they went the wrong way about it. This propeller, with this driving power behind it, practically gives the vertical ascent, especially when once flying speed has been obtained." "Blitz, but it is wonderful!" concluded Max, his enthusiasm growing by leaps and bounds, as he continued his examination. "Why, the propellers are made of steel, and so are the planes," exclaimed Carl, who was now carefully examining the material of which the aeroplane was made. "Steel, tempered steel, every bit of it--fuselage, propellers, tail fin, rudders. There’s not an ounce of wood about the _Scorpion_," returned the mentor. "Then the danger of fire is lessened," ventured Max, whose one dread in the air had always been that of fire. "That danger is eliminated," replied the chief, in a tone of certitude. "Except by petrol. By the way, where are the petrol tanks?" exclaimed Carl, who had never missed them till now. "There aren’t any," replied the Rittmeister, smiling. "I was waiting for that question." "No petrol tanks?" came the astonished cry from both the airmen at once. "They’re not necessary," returned the other; "and that’s the greatest mystery of all." "Himmel! Am I dreaming?" exclaimed Max. "No, you’re wide awake. Don’t stare like that, man!" "Der Teufel, but how is she driven?" demanded the scout, staring with wide-open eyes from Spitzer to the professor, and from the latter to his mechanic, who had stood by all this while, with arms akimbo, silently amused at the bewilderment of the two strangers. "Listen," began the Rittmeister. "I cannot explain everything now--time will not permit--but you shall learn all these things before many days are over." "Yes, go on!" "The professor has spent years on this series of inventions, both in the workshop and the laboratory, and each discovery has been co-ordinated and fitted into the scheme. The greatest of all his discoveries is the fact that he has been able to discover and to harness an unknown force to drive the motors of the _Scorpion_." "A highly compressed gas, I suppose," interposed Max, who had taken a science degree at Bonn. "Certainly, it is a _most_ highly compressed gas, extracted at great pains and labour from the elements. The formulæ for this wonderful new element exist only in the still more wonderful brain of the professor. It has not been committed to paper even, in its final terms and ratios, so that, even should this machine be captured, which it certainly shall not be whilst I am its pilot, it could not be used, once the present supply of this Uranis, as we will call it, is used up." "That is why the engines are so small, then?" ventured Max. "Precisely!" "And what is our present supply of this wonderful element?" "Do you see this?" said the Rittmeister, pointing to a few small cylinders, each about two feet long, and six inches in diameter, which lay carefully piled upon each other on the floor near the _Scorpion_. "Yes."