The evening journey was continued till a late hour, and the boys were surprised to find that the desert air, so scorching during the day, was of a chilliness suggesting frost at night. Doctor Bronson recalled to them their experience in India, where there was often a sensible change of temperature in going from sunshine into shadow, and said it was a noticeable feature of the desert that it did not retain at night any appreciable portion of the heat poured upon it during the day. "It is fortunate for man that such is the case," he added, "as the coolness of the night enables us to recuperate from the exhaustion of the sweltering temperature of daytime." [Illustration: SUNRISE ON THE SEA OF SAND.] The wells or springs to which allusion has been made are about half way from Korosko to Aboo Hamed. They are shallow pools of exceedingly bitter water, quite unfit for men to drink, but not injurious to camels. Doctor Bronson tasted the water, and said the bitterness was caused by sulphate of magnesia, commonly known as Epsom salts. Frank and Fred were curious to try it, but their curiosity was easily satisfied. A few drops on the tongue made a burning sensation, which did not show a disposition to go away immediately. The tents were pitched a short distance from the wells and close to an encampment of Arabs, who were spending two or three days there to refresh their camels. Around some of the pools there was a little vegetation, but not enough to furnish a good meal for a hungry animal; there were a few stunted palms in the valley, and the lines on the sand showed that at some former time a river flowed there. The camels drank freely of the water, and evidently understood that they must lay in a supply for the rest of the journey to the Nile. [Illustration: SCENE AT THE WELLS.] They left the wells early in the morning, and after a few hours found themselves on a broad, sandy plain, where the thermometer at two o'clock in the afternoon stood at 100°. It was the greatest heat they had found since leaving the Nile. Frank kept the record of the temperature, and reported to the Doctor each evening the result of his observations. In the morning it was chilly; the Arabs shivered in all their wrappings, and our friends sought shelter in their overcoats for the early part of the ride, but invariably laid them aside when the sun was a couple of hours above the horizon. By noon they were in their lightest garments, and so continued till evening, when the air grew cool again. There was a daily variation of not far from forty degrees between the highest and lowest readings of the thermometer. The lowest record was 50°, and the highest 100°; but these did not occur on the same day. The boys were not slow to understand why the Doctor had made such a liberal provision of blankets, and were greatly obliged to him for his forethought.