Abdul quickened the pace of his camel, and was soon at Frank's side.
"You told us there was no water on the road," said Frank, "and there is a lake right before us. I suppose it is filled with salt-water, and therefore doesn't count."
"Worse than salt-water," responded Doctor Bronson, who joined them. "It isn't water at all, but a mirage."
"Of course it is," Frank exclaimed, with a laugh; "I ought to have known better than be deceived by it."
"You are not the first to be deceived by it," said Abdul, "and it has been the cause of many deaths on this very route. Men have wandered from the road, confident that they were on the borders of a lake or river, and have fallen exhausted to die on the ground. Colonel Long, in his account of his travels in Central Africa, tells of a regiment on its march from Korosko across this desert. The men saw these lakes of water formed by the mirage; deceived by the illusion and maddened by thirst, they broke from the ranks, in spite of the protests of their guide, and went in search of water. They found their error too late, as most of them perished of exhaustion and thirst."
"Look at the graves along the road, and the bleaching bones of the camels," said the Doctor, "and you will understand the perils of the journey over the desert."
It was as Doctor Bronson suggested; the way was marked by thousands of skeletons of the faithful beasts of burden that had fallen in the service of their masters; and here and there, at painfully frequent intervals, were the graves of men who had perished of thirst or of the excessive heat. Even if no other landmarks existed, these would be sufficient to indicate the road.
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."