in half an hour, sir."
"Have you told them at Hounslow?"
"Yes, sir, and they have already got out the coloured lights and the ground flares."
"You have done well, Jones, but you had better not leave the office to-night. I'm very sorry, but I may want you. This is urgent business; we're up against something this time, and unless Keane and Sharpe have found something out, we're going to be beaten."
"I'll stay, sir, but what about you? This is your third night-sitting, and you've had nothing since lunch. Shall I order supper for you?"
"Oh, thanks, Jones, but I'd forgotten. Yes, you may order me coffee and a sandwich, and get something for yourself. You're getting the strain as well, and I don't want you to break down."
When left alone, Colonel Tempest once more began to pace the soft-carpeted room, much as a captain paces the bridge when his thoughts are unduly disturbed by some untoward event during the watch of the second officer. Every other minute he consulted his watch, and wondered why the time passed so slowly. Twice he rang down to the lobby attendant and asked if Captain Keane had arrived, and twice the same answer was returned.
Then he looked at the maps on the wall, and followed with his finger the trail of the All-Red Route which the aerial liners followed, linking up the empire and half the world. Now and again he would glance shrewdly at the large map of Germany, as a skipper eyes the weather quarter when a storm is brewing. Occasionally he would murmur half aloud:–
"A silent engine … three hundred miles an hour. Gee whiz! but they have beaten us two to one. We shall never catch them."
Then a slight sound caught his ears from outside the great building. The soft purr of an approaching Rolls-Royce motor and the sharp blast of a Klaxon horn followed.
"At last!" he cried. "Here they come!"
CHAPTER IV
A MIDNIGHT CONSULTATION
The next moment the door burst open and two men in flying helmets and leathern coats entered the room, and saluted the Colonel. Without any ceremony the latter greeted them warmly, almost joyously, for their cheerful presence gave almost instant relief to his over-burdened mind.
"Good evening, Keane. Good evening, Sharpe," he exclaimed, stepping forward and gripping each of them warmly by the hand.
"Good evening, sir."
"Now, have you discovered anything?" began the chief, without waiting for them to divest themselves of their heavy gear.
Keane looked at the Commissioner for a second or two and then answered:–
"Yes, and no, Colonel."
"H'm. That means something and nothing, I presume."
"Exactly, sir," continued Keane, who acted the part of spokesman. Then, speaking more solemnly, and in lowered tones, he continued, "We are up against something abnormal; I had almost said something supernatural. When you recalled us we were hot on the trail of the man who, in my opinion, is behind this conspiracy."
"You mean this Professor Weissmann?" added the chief of the aerial police.
Keane nodded.
"I thought so. This man is evidently an evil genius of very high mental calibre, and he has determined, out of personal revenge for the defeat of Germany, to thwart the Allies, and in particular Great Britain."
"He is a master-mind, and a highly dangerous personality; dangerous because he is so clever. And now that he has secured a few daring airmen for his tools, there is no end to the possibilities which his evil genius may accomplish before he and his crew are run to earth," replied Captain Keane.
"I know it, I know it–look here!" and the colonel handed him the batch of cables and wireless messages which showed how the Scorpion had already got to work.
"H'm! and there will be worse to follow," added the airman after he had glanced through the list.
"Now, tell me briefly what you have found, Keane, after which we must get to work to devise some immediate plan to thwart these aerial brigands. But first take off your flying gear, and sit by the fire, for you must be hungry, tired and numbed after that cold night ride." Then, ringing for his attendant, he ordered up more strong coffee and sandwiches.
"Thanks, Colonel, I will not refuse. It was indeed a cold ride, and we had no time to get refreshments before leaving the aerodrome at Cologne this evening," said Sharpe, as he divested himself of his heavy gear, sat by the fire and enjoyed the coffee which soon arrived.
A few moments later, the three men were engaged in serious conversation, although the hour of midnight had long since been tolled out by Big Ben.
"You sent me," Keane was saying, "to discover the whereabouts of this great German engineer and man of science, this brain wave whose perverted genius is likely to cost us so dear."
"And you were unable to find any trace of him?" interposed the chief.
"Well, we were unable to come into contact with him, for we found that since peace was concluded he had vacated his professorial chair at Heidelberg University, where he had been engaged for some considerable time, not only on some mechanical production, but in an attempt to discover some unknown force, evidently a new kind of highly compressed gas to be used for propulsive purposes."
"Had he been successful?"
"That, it was impossible to find out during our short stay over there," replied Keane, "but I discovered from someone who had been in close touch with him just about the time peace was signed, that he had expressed himself in very hopeful terms."
"Was he a very communicative type of man, then, did you learn?"
"No; on the contrary, he seldom spoke of his work, but on this occasion, when he communicated this information, he was very much annoyed at the defeat of Germany, and considered that his country had been betrayed into a hasty peace."
"And what happened to him after that?" asked the colonel.
"Shortly afterwards he disappeared completely, taking with him all the apparatus connected with his research work, also a highly skilled mechanic who had been specially trained by him for a number of years. But he left not a trace of himself or his work," said the captain, pausing for a moment to light a cigarette.
"Do you think he is acting under any instructions from his authorities?"
"No, certainly not; he distrusts his present Government entirely, and considers them traitors to the Fatherland."
There was another brief silence, whilst the three men, wrapt in deep thought, sat looking into the fire, or watched the rings of tobacco smoke curling upwards to the ceiling. At last, Captain Sharpe observed:–
"A powerful intellect like that did not suddenly disappear in this way without some ulterior motive, Colonel Tempest."
"Obviously not," returned the latter briefly, for he was deep in contemplation, and his mind was searching for some clue. At length he turned to the senior captain and said:–
"This silent engine theory, Keane, what do you think of it?"
Keane shook his head doubtfully, and the colonel handed to him once more the recent wireless
