They had been traveling some four or five hours since leaving their camp when Bill heard the drone of an airplane engine. He stopped and listened. The stream they were following was deep down in a valley with high hills on both sides. He could not possibly see that plane unless it flew directly overhead. Bill thought that if he could get a glimpse of the plane he could form an estimate of his present location. It would not do much good in regard to cutting down the distance which they still had to go but it would satisfy his curiosity. The airplane apparently was still some distance away.