There had been instant acceptance of the theory that the storehouse and magazine had been deliberately blown up by the cunning contrivance of a spy or spies within the city. Every stranger must give an account of himself, and even some individuals here and there who were not newcomers. Billy and Henri could see no opening where two full length military biplanes could alight without notice, and not a morsel of encouragement to try for negotiation on the quiet with the disguised secret agent who had summoned them. But the aviators hung about, not knowing what else to do for the present, thinking that Roque would make a showing of some sort, as he usually did in tight places. Flying lower and lower, the two biplanes were sweeping within earshot of the terraced heights along the river front, and though now of dim vision, searching parties could be seen flashing lights up and down the ground tiers. There was a hullabaloo breaking out on the lowest terrace, immediately overhanging the river--a shot--another and another--like a bunch of firecrackers, so fast did they follow! A stentorian note of defiance, a rush, two shapes springing out into space, a great splash in the icy waters below! If the morning revealed a single trace of the daring fugitives dead or alive, no word of it reached the aviation camp, to which the young airmen had returned, conscious that of this mission they were acquitted. "Do you know, I can't help believing that they got across?" Henri had a thought, perhaps, of the rabbit's foot that Schneider carried. The boys had many under-the-breath discussions as to the possible connection of Roque with the explosion that had destroyed the war depot. They had no reckoning that in the little shop of a silversmith, not far removed from the very column and statue that had twice served them as a guide-post, the whole story might have been told by a wily confederate posing as a peaceful artisan. This same man could also have confessed to the first error of his expert career in the handling of a time-clock. With plots and counterplots, however, the young aviators had no time or inclination to meddle. They would rather work in the open. "I wonder if that lieutenant meant what he said about giving us a peek at Petrograd?" Billy put the question to his chum as they contemplated with satisfaction a particularly neat job of aëroplane repair they had just completed. "Don't see why he should say it if he did not mean it," replied Henri. "Next time he comes this way there would be no harm in reminding him of what he said." It so happened that the aviation chief at the very moment was headed for the hangars. He was accompanied by two officers of apparent high rank, who gave the various types of aircraft close and critical inspection. When the No. 3's came to their notice, one of the officers, a grizzled veteran, with a livid scar showing from temple to chin, halted with a pointed word of commendation.