"Stop! I will come. The Tsar has as good servants as I am! Colonel, tell his Majesty that I gave myself up to save the lives of better men." Then the Professor mounted the ladder amidst a murmur of relief and applause from the crowd, and, gaining the deck of the _Ariel_, bowed coldly to Mazanoff and said-- "I am your prisoner, sir!" The captain of the _Ariel_ bowed in reply, and stamped thrice on the deck. The fan-wheels whirled round, and the air-ship rapidly ascended, at the same time moving diagonally across the quadrangle of the Arsenal. Scarcely had she reached the other side when there was a tremendous explosion in the north-eastern angle of the building. A sheet of flame shot up through the roof, the walls split asunder, and masses of stone, wood, and iron went flying in all directions, leaving only a fiercely burning mass of ruins where the gable had been. The Professor turned ashy pale, staggered backwards with both his hands clasped to his head, and gasped out brokenly as he stared at the conflagration-- "God have mercy on me! My laboratory! My assistant--I told him"-- "What did you tell him, Professor?" said Mazanoff sternly, grasping him suddenly by the arm. "I told him not to open the other cylinder." "And he has done so, and paid for his disobedience with his life," said Mazanoff calmly. "Console yourself, my dear sir! He has only saved me the trouble of destroying your laboratory. I serve a sterner and more powerful master than yours. He ordered me to make your experiments impossible if it cost a thousand lives to do so, and I would have done it if necessary. Rest content with the knowledge that you have saved, not only the rest of the Arsenal, but also Petersburg, by your surrender; for sooner than that secret had been revealed, we should have laid the city in ruins to slay the man who had discovered it." The prisoner of the Terrorists made no reply, but turned away in silence to watch the rapidly receding building, in the angle of which the flames were still raging furiously. A few minutes later the _Ariel_ had rejoined her consorts. Her captain at once went on board the flagship to make his report and deliver up his prisoner to Natas, who looked sharply at him and said-- "Professor, will you give me your word of honour to attempt no communication with the earth while it may be found necessary to detain you? If not, I shall be compelled to keep you in strict confinement till it is beyond your power to do so." "Sir, I give you my word that I will not do so," said the Professor, who had now somewhat regained his composure. "Very well," replied Natas. "Then on that condition you will be made free of the vessel, and we will make you as comfortable as we can. Captain Arnold, full speed to the south-westward, if you please." CHAPTER XXVIII. A SKIRMISH IN THE CLOUDS.