But all such considerations were quite remote from his thoughts as he and Tar-gash were crossing a rolling, tree-dotted plain in their search for the tribe of men to which the Sagoth was guiding him. By comparison with other plains they had crossed, this one seemed strangely deserted, but the reason for this was evident in the close-cropped grass which suggested that great herds had grazed it off before moving on to new pastures. The absence of life and movement was slightly depressing and Tarzan found himself regretting the absence of even the dangers of the teeming land through which they had just come. They were well out toward the center of the plain and could see the solid green of a great forest curving upward into the hazy distance when the attention of both was attracted by a strange, droning noise that brought them to a sudden halt. Simultaneously both turned and looked backward and up into the sky from which the sound seemed to come. Far above and just emerging from the haze of the distance was a tiny speck. "Quick!" exclaimed Tar-gash. "It is a thipdar," and motioning Tarzan to follow him he ran swiftly to concealment beneath a large tree. "What is a thipdar?" asked Tarzan, as the two halted beneath the friendly shade. "A thipdar," said the Sagoth, "is a thipdar;" nor could he describe it more fully other than to add that the thipdars were sometimes used by the Mahars either to protect them or to hunt their food. "Is the thipdar a living thing?" demanded Tarzan. "Yes," replied Tar-gash. "It lives and is very strong and very fierce." "Then that is not a thipdar," said Tarzan. "What is it then?" demanded the Sagoth. "It is an aeroplane," replied Tarzan. "What is that?" inquired the Sagoth. "It would be hard to explain it to you," replied the ape-man. "It is something that the men of my world build and in which they fly through the air," and as he spoke he stepped out into the opening, where he might signal the pilot of the plane, which he was positive was the one carried by the O-220 and which, he assumed, was prosecuting a search for him. "Come back," exclaimed Tar-gash. "You cannot fight a thipdar. It will swoop down and carry you off if you are out in the open." "It will not harm me," said Tarzan. "One of my friends is in it." "And you will be in it, too, if you do not come back under the tree," replied Tar-gash. As the plane approached, Tarzan ran around in a small circle to attract the pilot's attention, stopping occasionally to wave his arms, but the plane sped on above him and it was evident that its pilot had not seen him. Until it faded from sight in the distance, Tarzan of the Apes stood upon the lonely plain, watching the ship that was bearing his comrade away from him. The sight of the ship awakened Tarzan to a sense of his responsibility. He realized now that someone was risking his life to save him and with this thought came a determination to exert every possible effort to locate the O-220.