The passage of the plane opened many possibilities for conjecture. If it was circling, which was possible, the direction of its flight as it passed over him would have no bearing upon the direction of the O-220, and if it were not circling, then how was he to know whether it was traveling away from the ship in the beginning of its quest, or was returning to it having concluded its flight. "That was not a thipdar," said Tar-gash, coming from beneath the tree and standing at Tarzan's side. "It is a creature that I have never seen before. It is larger and must be even more terrible than a thipdar. It must have been very angry, for it growled terribly all the time." "It is not alive," said Tarzan. "It is something that the men of my country build that they may fly through the air. Riding in it is one of my friends. He is looking for me." The Sagoth shook his head. "I am glad he did not come down," he said. "He was either very angry or very hungry, otherwise he would not have growled so loudly." It was apparent to Tarzan that Tar-gash was entirely incapable of comprehending his explanation of the aeroplane and that he would always believe it was a huge, flying reptile; but that was of no importance--the thing that troubled Tarzan being the question of the direction in which he should now prosecute his search for the O-220, and eventually he determined to follow in the direction taken by the airship, for as this coincided with the direction in which Tar-gash assured him he would find the tribe of human beings for which they were searching, it seemed after all the wisest course to pursue. The drone of the motor had died away in the distance when Tarzan and Tar-gash took up their interrupted journey across the plain and into broken country of low, rocky hills. The trail, which was well marked and which Tar-gash said led through the hills, followed the windings of a shallow canyon, which was rimmed on one side by low cliffs, in the face of which there were occasional caves and crevices. The bottom of the canyon was strewn with fragments of rock of various sizes. The vegetation was sparse and there was every indication of an aridity such as Tarzan had not previously encountered since he left the O-220, and as it seemed likely that both game and water would be scarce here, the two pushed on at a brisk, swinging walk. It was very quiet and Tarzan's ears were constantly upon the alert to catch the first sound of the hum of the motor of the returning aeroplane, when suddenly the silence was shattered by the sound of hoarse screeching which seemed to be coming from a point further up the canyon. Tar-gash halted. "Dyal," he said. Tarzan looked at the Sagoth questioningly. "It is a Dyal," repeated Tar-gash, "and it is angry." "What is a Dyal?" asked Tarzan. "It is a terrible bird," replied the Sagoth; "but its meat is good, and Tar-gash is hungry."