Shortly before seven o'clock on this eventful evening, Billy and Henri had the biplanes in order for the arranged visit to St. Michael road, and the delivery of one or both of the machines to their former owner, supposedly in waiting in the shadow of the two towers. "It is really a relief that the time is drawing nigh for us to get off the rack. I believe we are doing the square thing, but sure we have had few easy moments during these last forty-eight hours." Billy heaved a sigh when reviewing this disturbing experience. Henri turned just then to salute the aviation chief. The boy's greeting had none of the cheery note usually there. He did not know how it would be several hours hence. "Looks like a chance for you boys on the next dispatch trip to Petrograd," advised the chief; "we can illy spare more than two at a time of our regular air scouts, and here's a deal by which we have two extra machines and a pair of pilots thrown in." With their minds clear and no cloud like the one looming ahead, the prospect of biplaning to the wonder city of Russia would have set the boys on the top floor of enthusiasm. As it was, they could only say that they would welcome the work if it should be assigned to them. The aviation chief had hardly taken a dozen steps in his continuing round of inspection when there was a shakeup that might have come by a combination of volcano and earthquake. "Geeminy!" gasped Billy, clapping his hands to his ears, "somebody must have fired a ton of powder!" A roll of drums preceded the hasty assembling of several regiments in this division, and a squadron of cavalry jingled madly down the street. "That was a whopper, all right," exclaimed Henri, righting himself after his first little stagger from the shock, "but big noises ought not to queer us, pard. Get in and get away." Following his chum's example, Billy was close behind the former in upward flight. They could see that the streets below held literally surging masses of humanity, all trending in the same direction. The aviators speedily gained an idea of what had happened. That which only the other day they had observed as a solid front of granite and iron on a building covering practically a whole city square had fallen in ruins, completely blockading the broad avenue it had faced. About the square a cordon had been drawn, and it could be seen, even through the dusk, that troops were spreading fan-shape from this point throughout the entire northern section, while the police darted right and left and everywhere. The select neighborhood of St. Michael road had not been omitted from the general round-up, the boys found, when they approached the site of the two towers. It seemed that the abandoned team and wagon had been found somewhere along the highroad, and as suspicion was now acute, the discovery set the fine-comb going along every terrace and police poking in every likely hiding-place.