“I am thinking of him, and I wish to think yet a little longer,” quietly spoke the professor, “keep a lookout, lads, and if you see aught of Waldo's fair women, pray notify me.” For the better part of an hour comparative silence reigned, the boys feasting eyes upon yonder spectacle, their uncle deeply in reverie; but then he roused up, his final decision arrived at. “I will do it!” were his first words. “Yes, I will do it!” “Do what, uncle Phaeton?” asked Waldo, with poorly suppressed eagerness, as he turned towards his relative. “Go after Cooper Edgecombe,--bringing him here in order that he may, sooner or later, solve this perplexing enigma. Come, boys, we may as well start back towards the aerostat.” But both youngsters objected in a decided manner, Waldo saying: “No, no, uncle Phaeton! Why should we go along? You'll be coming right back, and will be less crowded in the ship if we don't go.” “And we can better wait right here; don't you see, uncle?” “To keep the Lost City safely found, don't you know? What if it should take a sudden notion to lose itself again?” added Waldo, innocently. CHAPTER XVII. AN ENIGMA FOR THE BROTHERS. In place of the indulgent smile for which he was playing, Waldo received a frown, and directly thereafter the professor spoke in tones which could by no possibility be mistaken. “Come with me, both of you. I am going back to the aerostat, and I dare not leave you boys behind. Come!” Kind of heart and generally complaisant though uncle Phaeton was, neither Bruno nor Waldo cared to cross his will when made known in such tones, and without further remonstrance they followed his lead, slipping away from the snug little observatory without drawing attention to themselves from any of yonder busy horde. Not until the trio was fairly within the gulch did the professor speak again, and then but a brief sentence or two. “Give me time to weigh the matter, lads. Possibly I may agree, but don't try to hurry my cooler judgment, please.” Waldo gave his brother an eager nudge at this, gestures and grimaces being made to supply the lack of words. But when, the better to express his confidence that all was coming their way, the youngster attempted a caper of delight, his foot slipped from a leaf-hidden stone, and he took an awkward tumble at full length. “Never touched me!” he cried, scrambling to his feet ere a hand could come to his aid. “Who says I don't know how to stand on both ends at the same time?” Barring this little caper, naught took place on their way to the air-ship; and once there, the professor heaved a mighty sigh, wiping his heated face as one might who has just won a worthy race. But he betrayed no especial haste in setting the flying-machine afloat and Waldo finally ventured: “Can we help you off, uncle Phaeton?” But he was assured there existed no necessity for such great haste.