"Some experiment," I stammered at length. "Professor Chang is at work upon some problem." Morse shook his head. "Not he! I'll swear that old Chang would never be letting off fireworks without consulting or warning Pu-Yi. Kirby, there is some black business stirring! We must look into this. I don't like it at all--hark!" He suddenly stopped speaking, and put his hand to his ear. His whole face was strained in an ecstasy of listening, which cut deep gashes into that stern, gnarled old countenance. I listened also, and with dread in my heart. Instinctively and without any process of reasoning, I knew that in some way or other the horror was upon us again. My lips went dry and I moistened them with the tip of my tongue; and, without conscious thought, my hand stole round to my pistol pocket and touched the cold and roughened stock of an automatic Webley. Then I heard what Morse must have heard at first. The air all around us was vibrating, and swiftly the vibration became a throb, a rhythmic beat, and then a low, menacing roar which grew louder and louder every second. We had turned to each other, understanding at last, and the same word was upon our lips when the thing came--it happened as rapidly as that. Skimming over the top of the distant Palacete like some huge night-hawk, and with a noise like a machine gun, came a venomous-looking, fast-flying monoplane. It swept down into Grand Square like a living thing, just as the noise ceased suddenly and echoed into silence. It alighted at one end and on the side of the fountain nearest the observatory, ran over the smooth wood-blocks for a few yards, and stopped. It was as though the hawk had pounced down upon its prey, and every detail was distinct and clear in the brilliant light of the lamps in the Square below. Both of us seemed frozen where we stood. I know, for my part, all power of motion left me. A choking noise came from Morse's throat, and then we heard a cry and from immediately below us came the figure of Pu-Yi, hurrying down the library steps and running towards the aeroplane, which was still a considerable distance from him. The next thing happened very quickly. A door at the foot of the observatory tower opened, and out came what we both thought was the figure of the astronomer. He was a tall, bent, old man, habitually clothed in a padded, saffron-colored robe with a hood, something like that of a monk. "Chang!" I said in a hoarse whisper, when Pu-Yi stopped short in his tracks, lifted his arm, and there was the crack of a pistol. The figure beyond, which was hurrying towards the monoplane, swerved aside. The robe of padded silk fell from it and disclosed a tall man in dark, European clothes. He dodged and writhed like an eel as Pu-Yi emptied his automatic at him, apparently without the least result. Then I saw that he was at the side of the aeroplane, scrambling up into the fuselage assisted by the pilot in leather hood and goggles.