Jack and Bob were both six feet tall, and Bob in addition was possessed of extraordinary strength. As for Frank, an orphan, who made his home with Bob on the Temple estate, adjoining that of the Hamptons’ near Southampton, Long Island, what he lacked in inches and girth, was made up in quickness of intellect. All three were students at Yale. This was the way matters stood, with the party at length after its trip across the Sahara from Khartum drawing near the Oasis Aiz-Or, when Jack paused to await the approach of his comrades. As Frank drew nearer, Jack smiled. He was thinking of the other’s comical appearance. Wrapped in the voluminous jillab which all wore as it provided greater protection against sand and heat than European clothing, Frank was crowned by a sun helmet, startling by contrast, and beneath it wore headphones clamped over his ears. Jack was on the point of calling out some laughing remark about the latter’s vain wait for a message from Professor Souchard, when Frank’s face suddenly betrayed alarm. And with a shout he tore the headset from his ears, sending the sun helmet spinning out on the floor of the desert. Turning about, he beckoned wildly for Mr. Hampton and Ali to approach. “What is it?” shouted Jack. “What did you hear?” For, that Frank had received some message filling him with alarm was apparent. Frank did not reply. His face grew pale beneath the heavy tan. CHAPTER II. A CRY FOR HELP. The long rays of the setting sun, which almost touched the horizon, were flung across the desert, turning it into dazzling gold, as Mr. Hampton and Ali pushed their camels close to where the three boys had come together. The camels stood with feet spread apart, seemingly asleep. Jack and Bob, who also had drawn close, were bombarding Frank with questions and, almost inarticulate at first, he had just begun to answer when Mr. Hampton and Ali arrived. In the background crowded the half dozen Arab guards, sensing something amiss. “A cry for help,” Mr. Hampton heard Frank say. “The Professor was sending out an appeal to us.” Frank looked wildly around at the group. “Great Scott, can’t we do something?” he appealed. “Calm down, Frank,” said Mr. Hampton. “Tell us about it, and then we can decide what to do.” Frank nodded as he got a grip on his emotions. “Well, maybe, I was a little inarticulate,” he said, with a rueful smile. “But, just think. Here I was, bumping along on my camel, and half asleep. I had the headpiece on, the phones to my ears. But I hadn’t any real idea I’d hear anything. What’s there to hear, way out here, away from all the world? The only chance was that Professor Souchard would take a notion to broadcast something for our benefit. “Then it happened.” He paused and looked at the others, before swallowing and resuming, with his face still pale. “Suddenly I heard the Professor’s voice, just as if he were right out there on the desert.”