When we had talked it over, he gave me a cigar and we began to promenade the whole length of the roof. As I have said, the night was clear and calm. Again the great stars globed themselves in heaven with an incomparable glory unknown and unsuspected by those down below. The silence was profound, the air like iced wine. From where we were, we had a bird's-eye view of the whole City. Grand Square lay immediately at our feet, brilliantly illuminated as usual. Not a living soul was to be seen; only the dragon-fountain glittered with mysterious life. To the right, beyond the encircling buildings of the Square, stood the Palacete Mendoza surrounded by its gardens, a square, white, sleeping pile. I sent a mental greeting to Juanita. So high was the roof on which we stood that only one of the towers or cupolas rose much above us. It was the dome of the observatory, exactly opposite on the other side of Grand Square. "There is some one who isn't much troubled by sub-lunary affairs," I said, pointing over the _machicolade_. Morse nodded, and expelled a blue cloud of smoke. "I guess old Chang is the most contented fellow on earth," he said. "He is Professor, you know, Professor Chang, and an honorary M.A. of Oxford University. I had him from the Imperial Chinese Observatory at Pekin, and I am told he is on the track of a new comet, or something, which is to be called after me when he has discovered it--thus conferring immortality upon yours truly! "It is an odd temper of mind," he went on more seriously, "that can spend a whole life in patient seclusion, peering into the unknown, and what, after all, is the unknowable. Still, he is happy, and that is the end of human endeavor." He sighed, and with renewed interest I stared out at the round dome. The slit over the telescope was open, which showed that the astronomer was at work. In the gilded half-circle of the cupola, it was exactly like a cut in an orange. I was about to make a remark, when an extraordinary thing happened. Without any hint or warning, there was a loud, roaring sound, like that of some engine blowing off steam. With a "whoosh," a great column of fire, like golden rain, rose up out of the dark aperture in the dome, towering hundreds of feet in the sky, like the veritable comet for which old Chang was searching, and burst high in the empyrean with a dull explosion, followed by a swarm of brilliant, blue-white stars. Some one inside the observatory had fired a gigantic rocket. Morse gave a shout of surprise. He had a fresh cigar in his hand, and, unknowingly, he dropped it and mechanically bit the end of his thumb instead. "What was that?" I cried, echoing his shout. He didn't answer, but grew very white as he stepped up to the parapet, placed his hand upon the stone, and leant forward. I did the same, and for nearly a minute we stared at the white, circular tower in silence. Nothing happened. There was the black slit in the gold, enigmatic and undisturbed.