"Still, public opinion had to be satisfied, and you sent the patrols where the public demanded that they should be sent. Besides, if you recall them now, this raider will probably pick up your messages and change his tactics. I can tell you this, Colonel, that while he can get his necessary supplies of uranis, and a few extra spares from the workshop here, this von Spitzer intends to carry out his mad policy of destroying the civilized world by piecemeal. It is all part of a great plan to save Germany from the evil consequences of the Peace terms. But, whilst we hold this citadel, and retain these two men captive, his activities are limited to his present supply of this secret element--uranis." The colonel swore under his breath, and went to examine the prisoners, to make sure that there was no chance of their escaping, for he felt the truth of Keane’s words. He now felt grateful that the airman had not responded to the message for his recall, although it had amounted to a serious breach of discipline. "Ah, well," he said at length, "it only remains to capture this raider, and the whole system of their clever and daring attempt to convulse the Allies, break up their international system of mail transit, stop the intercourse of civilized nations, and cause a world revolution--all these things will fail." So their efforts were redoubled to make preparations to capture the wonder ’plane, should it descend on the aerodrome. A couple of machine guns were found, and mounted, under the charge of Sharpe and Captain Hooper, though the skipper of the airliner pointed out that the _Scorpion_ carried bullet-proof armour. "You will need to hit her in a vital spot," he said, "so that your first burst may be your last, or she will be up again like a helicopter." "Then we must have the two Bristols ready," urged the colonel, "though it’s a deuce of a hole to get out of with this new type of a Bristol Fighter." "And the petrol, sir?" asked Keane, who, was rather anxious on this point, for he hoped that the _Scorpion_ would become his victim in the coming air fight. "There may be sufficient for another two hours, certainly not more." "That means unless the _Scorpion_ chooses to stay and fight, she’ll simply leave us." "Von Spitzer will fight unless I stop him!" called out the professor from behind the curtains, where he was confined under the charge of his colleague of other days, for he had been listening to the conversation. "So much the better!" replied Keane, tartly. "And when the fight is over there won’t be many of you left alive to tell the story," came the rejoinder. *CHAPTER XXII* *AN AERIAL DUEL* "Message from the _Scorpion_, sir!" cried Keane, a little before midday, from the little key-board where he had been patiently waiting for the last hour. "Good! What does the brigand say?" asked Tempest. "Expects to be here within an hour."