"Curl up tightly in your blankets," advised Mr. Lewis, as he prepared to occupy a corner of the tent. "It will be very cold along toward dawn." "And will we sleep!" said Bob, throwing himself down in relaxation. Bob's prediction proved right. The boys enjoyed the deep slumber that only an explorer can experience. It was wholly unexpected that they be awakened at dawn by their elders. "It's a dirty trick!" grinned Joe, as his father urged him to fully awaken. "I know," the naturalist said sympathetically. "But we must be on our way before the sun gets so hot that traveling will be dangerous." But once up, it was easy to forget sleepiness and enjoy the early-morning air. "Talk about cold!" cried Joe, shivering in spite of the fact that he was wrapped in a heavy coat. "I'll bet the old mercury is down to thirty degrees." "Not quite that bad," laughed Dr. Kirshner, holding up a thermometer. "It's thirty-eight." "Even that is cold for the desert," said Bob. "I always had the impression that it is hot all the time." "There are some places that way," returned Mr. Lewis. "But certainly not the old Sahara." Mr. Lewis and Mr. Holton got out the provisions and began preparing the breakfast. "And while they're doing that, suppose we go up to the top of this mound," suggested Bob to his chum. "It's rather high. Maybe we can get a view of the country ahead of us." "Don't be gone long, boys," called Dr. Kirshner, as they made their way to the edge of the hill. The sand under their feet was, in many places, loose and unstable, but they found sufficient hard spots to make safe footholds. It was not a little difficult to estimate where they could safely climb higher. Perspiration was dripping from them when they at last reached the top and turned their gaze toward the horizon. A cry of astonishment came from Joe at the scene that was spread out before them. The dunes that stretched away in the distance were tinted with many colors, showing strangely in the early-morning light. Some were pink, others blue, still others green, while the ones farther away paled away into nothingness. "That's a new one on me," remarked Bob, highly puzzled. "I never expected anything like this." "Nor I. Wonder what causes it?" "Beyond me." They stood for some time gazing at the varicolored mounds. Then Joe turned about. "Better get back to camp," he suggested. "Breakfast may be ready." They were about to retrace their footsteps when they heard a rustling sound but a few feet away. "What's that?" muttered Bob, sensing that danger was at hand. The youths glanced around--and then shrank back in horror! CHAPTER XIII The Hideous Reptile "A horned viper!" exclaimed Bob fearfully, recognizing the long, sinister body that was coiled almost at their feet. "And it's going to strike!" cried Joe. "Get your pistol--quick! We'll both fire at it. Oh, how I wish we had our rifles!"