The air-ship gave an awkward lurch just then, and down went the little professor to thump his head heavily against one corner of the locker. Swaying drunkenly from side to side, then tossing up and down, turning in unison with those fiercely whirling clouds, the aeromotor seemed at the point of wreck and ruin. Desperately the trio clung to the life-lines, clenching teeth upon the life-giving tubes as that terrible pressure increased so much that it seemed impossible for the human frame to longer resist. Fortunately that ordeal did not long endure, and again relief came to those so sorely oppressed. A brief gasping, sighing, stretching as the aerostat resumed its level position, merely rocking easily within that partial vacuum, and then Waldo huskily suggested: “Looks like the blame thing was sick at the stomach!” No doubt this was meant for a feeble attempt at joking, but Professor Featherwit took it for earnest, and made quick reply: “That is precisely the case, my dear lad, and I am greatly joyed to find that you are not so badly frightened but that you can assist me in taking notes of this wondrous happening. To think that we are the ones selected for--” “I say, uncle Phaeton.” “Well, my lad?” “If this thing is really sick at the stomach, when will it erupt? I'd give a dollar and a half to just get out o' this, science or no science, notes or no notes at all!” “Patience, my dear boy,” gravely spoke the little man of science, busily studying those eddying currents like one seeking a fairly safe method of extrication from peril. “It may come far sooner than you think, and with results more disastrous than feeble words can tell. We surely are a burden such as a tornado must be wholly unaccustomed to, and I really believe these alternations are spasmodic efforts of the cloud itself to vomit us forth; hence you were nearer right than you thought in making use of that expression.” Just then came a rush of icy air, and Bruno pantingly cried: “I'm swelling up--like Aesop's--bullfrog!” CHAPTER IV. THE PROFESSOR'S LITTLE EXPERIMENT. Again those involuntary riders of the tornado were tossed violently to and fro in their seemingly frail ship, while the balloon itself appeared threatened with instant dissolution, those eddying currents growing broken and far less regular in action, while the fierce tumult grew in sound and volume a thousandfold. All around the air-ship now showed ugly debris, limbs and boughs and even whole trunks of giant trees being whirled upward and outward, each moment menacing the vessel with total destruction, yet as frequently vanishing without infringing seriously upon their curious prison. Sand and dirt and fragments of shattered rock whistled by in an apparently unending shower, only with reversed motion, flying upward in place of shooting downward to earth itself.