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."

"That is the world's supply at present, excluding the two cylinders which are already fitted on the machine."

"The world's supply," ejaculated Carl, who was thinking of the huge petrol tank, which in a Fokker scout would last only three hours with the throttle wide open. "That won't last long, unless the pressure is enormous."

"The pressure is enormous, my friend; so enormous that if anything happened it would----"

"Blow a hole in the universe, I reckon," interposed Max.

"You are right, and that is the only danger connected with the Scorpion. The other danger you mentioned, that of fire, is altogether eliminated. There would be nothing to burn if one of these cylinders exploded, for there would be nothing left--in the vicinity."

"Sacre bleu!" exclaimed Carl, sotto voce, for, brave youth that he was, he shuddered at the thought.

Max was the more practical of the two, however, for he belonged not to the highly sensitive scouts, but to the heavy bombers, and he merely asked to satisfy his curiosity:--

"How far will one of those cylinders take us, Rittmeister?"

"Ten thousand miles," replied the chief, "that is, one fitted to either engine."

"Good! Let me see, there are ten here, and one already fitted to either motor makes a dozen. Why, they would carry us"--and here he made a rapid calculation--"they would take us twice round the world."

"Precisely, and with a little to spare, when we had completed the double trip."

"And what speed would she pick up, say at a level flight?"

For answer the chief looked at the professor, as though uncertain whether to reply to this question.

"They have taken the oath, sir," he pleaded, "They cannot withdraw," and the great scientist nodded his acquiescence.

"Two hundred and fifty miles without being pushed," he replied at length.

"Donnerwetter! And what if she were pushed?"

"I cannot say, she has never been driven beyond that."

"What a deuce of a noise she will make--like a whole formation of Gothas, I should imagine," said Max.

The professor smiled, but left it to the Rittmeister to explain this last point.

"The engines are silent, but there is a slight hum from the propellers. That cannot be effaced at present, but it is nothing."

Then, having given all these details, the visitors made a closer inspection of the machine. They were permitted to climb into the conning-tower, to handle the controls, and the two swivel machine guns mounted there. They were shown into the little cabin, where four men might sit at the little table, or lie down at full length, but could not stand upright. The steel struts, steel folding wings, the carefully packed spares, the little mica windows in the cabin--these, and a dozen other things, were pointed out and explained to them--the stores which were already packed, comprising chronometrical instruments, maps, charts, ammunition for the guns, compressed food, etc., until their bewilderment grew, and their astonishment became unbounded.

"Why, she scarcely needs an aerodrome at all!" Carl ventured at length.

"Scarcely," replied the chief. "At any rate, not for a long time."

"She is weather proof; she is wonderfully camouflaged. She could hide in a desert, or a meadow," said Max.

"And she carries her own stores for a long, long trip," ventured Carl, who was just dying for the morrow to come.

"And if she were chased, she could make rings round anything, even a Fokker scout, or a verdammt British S.E.5," added Max.

"So you are satisfied, both of you?" asked the Rittmeister.

"Perfectly satisfied. I am only longing for to-morrow, so that I may turn aerial brigand, buccaneer, or what you like," answered Carl.

"And you, Max?"

"I am ready, chief, to follow you to the end of the world, for mine eyes have seen the wonder 'plane."

CHAPTER III

"TEMPEST" OF THE AERIAL POLICE

Colonel John Tempest, D.S.O., M.C., etc., late of the Royal Air Force, and now Chief Commissioner of the British Aerial Police, sat before a pile of papers in his office at Scotland Yard late