"Look at the eyes," said Joe. "They're almost jet black." The naturalists began at once the task of removing the skins, for it was best not to leave them on too long under such a sun. As soon as this was completed they were placed in a solution of alum that had been prepared by Dr. Kirshner under the naturalists' directions. "In our opinion, that's the best preservative there is," remarked Mr. Holton, "although there are many others in use." It required the better part of two hours to complete the task. The sun was just beginning to sink behind the distant mounds when they were ready to continue the journey. "Be dark before long," predicted Fekmah. "Then we will have to stop and wait for the moon. If you have not before traveled on desert under moon, you have much to see." They plodded continually on, Bob and Joe occasionally taking motion pictures. As the heat gradually became less intense, the youths felt a thrill of exultation run through their veins. Now there was nothing to worry about, no fierce sun to sweat under, no cares or anxieties to occupy their minds. Life--life on the boundless Sahara--seemed great. "The folks back home are sure missing something," smiled Bob. "Though I suppose a lot of them wouldn't care for doings of this kind." Slowly the darkness came. At last, when the little caravan was in a notch between two unusually high dunes, Tishmak called a halt and conversed with Fekmah. Then the latter turned to the Americans. "Tishmak says it is not thing to go on through darkness," he told them. "Might come on to something bad. We'll wait here for moon to come out, then go on." "Now's a good chance to get our suppers," said Dr. Kirshner, commanding his dromedary to lower. By the aid of flashlights a limited but satisfying meal was prepared on paper, and the adventurers all ate heartily. "Too bad there's nothing for the camels," said Mr. Lewis. "No need of worrying about them," returned Fekmah. "They can have no food or drink for good many days. Soon we come to region of thin vegetation. Then they eat." After the meal the explorers stretched out on the now cool sand, glad of a chance to rest their tired limbs. The constant jolting of the dromedaries had contributed much toward fatiguing them. "Now's the time when a coat comes in handy," said Bob, reaching into his bag on the camel's back. "Wait till dawn, if you want to see real cold weather," came from Dr. Kirshner. "Isn't that right, Fekmah?" The Arab nodded. "Temperature often gets down to thirty-five degrees," he said.