Only Professor Phaeton at once recognised the sound, through description, and each one of those swiftly succeeding strokes seemed falling upon his heart, bidding him mourn for his beloved nephew, upon whom his aged eyes had surely looked their last in this life! Yet it was the professor who took prompt action, speaking sharply as he darted across to where the air-ship rested: “Come; get aboard, and let us do what lies in our power. It was criminal to send the poor lad into the jaws of death, but now--hasten, there may be a chance, even yet!” The call was still hot upon his lips when his two companions entered the aerostat, gripping tight the hand-rail as Professor Featherwit sent the vessel afloat with reckless haste. As by a miracle they escaped disaster through rushing into a bushy treetop, and that fact served to steady the aeronaut's nerves. “On guard, uncle Phaeton!” cried Waldo, making a lucky snatch at his cap, which one of the stiff boughs brushed off his head. “Ay, ay, lad,” responded the man at the guiding-gear, as the air-ship shot onward and upward, now heading, as directly as was practicable, for the Lost City of the Aztecs. “That was the very lesson I needed. I am steady of nerve, now, and will show no lack,--heaven grant that we may not be for ever too late, though!” “What do you reckon could have kicked up such a bobbery, uncle? And what--ugh!” as the wardrum's throbbings again swelled forth in grim alarm. “What in time is that, anyway?” As briefly as might be, the professor explained, and almost for the first time Waldo felt a thrill of dread. “If they've got Bruno, what will they do with him?” That very dread was worrying uncle Phaeton, and already through his busy brain were flashing horrid pictures of punishment and sacrifice, of hideous scenes of torture, wherein the eldest son of his dead sister played a prominent role, perforce. He dared not trust his tongue to make answer, just then, and sent the aeromotor onward at top speed, leaning far forward to win the earliest glimpse of--what? He caught sight of blazing beacons fairly encircling the Lost City, forming a cordon through which no stranger could hope to pass unseen. He beheld hundreds of armed shapes rushing to and fro, plainly looking for some intruder or other enemy, yet almost as certainly failing as yet to make the longed-for discovery. Not until that moment had uncle Phaeton dared indulge in even the shadow of a hope. The awful alarm seemed proof conclusive that poor Bruno had been taken, through the treachery of Ixtli. Naturally enough, that was his first belief, but now, as the air-ship slackened pace to circle more deliberately above the valley, all eyes on the eager watch for either Bruno or something to hint at his fate, Professor Featherwit lost a portion of that conviction.