"I keep catching glimpses," replied Frank, with the glass to his eye; "but the whole thing seems to be dancing about.--Now I've got it.--No; gone again.--That's better. The vultures have hopped off the heap and are spreading their wings. We have scared them away. Yes, there they go--a few hops, and they are rising sluggishly. No, I can't follow them with the glass." "Can you see anything else?" "Yes, I've got the heap again, and there are three of the little dog-like creatures scurrying right away. I say, this is a good glass! I can see the dusty sand rise as it is kicked by the jackals. Here, let's stop the camel." "No," said the professor; "there's nothing worth stopping for." "But I want to make out something lying by that little heap. It looks like a curved bone." "It is a curved bone," said the professor. "You can't see with the naked eye." "No," said the professor, smiling; "but I have been along such a track as this before." "But there is no track," said Frank. "We are going over smooth sand, and making a fresh one." "Which will all be obliterated in a few hours. It is a track, though, as your heap proves." "I should have liked to examine it, though." "Well, you will have plenty of chance, for we shall go pretty close to it--but on the windward side." Frank lowered the glass to look inquiringly at the speaker. "Look here," he said; "you mean something by the way you just spoke." "Certainly I did." "What?" "Take your glass, and sharpen your powers of observation, my lad. The sooner you learn the desert the better for you." "I begin to have my suspicions," said Frank sharply. "If you wait a little longer, and go by there with your eyes shut, my lad, you will have something more than a suspicion." "Horrible!" said Frank shortly, as he once more raised his glass to his eyes. "You have given me the clue. I can make it out clearly now. Some poor camel that has strayed and lost its way, I suppose. Died from hunger and thirst." "More likely from old age or overwork," replied the professor; "a milestone, only one of the many that mark the caravan tracks across the desert. Some one must have passed here within forty-eight hours." "How do you know?" "By the appearance of that milestone. If we came by here to-morrow there would be nothing visible but some whitening bones. Look yonder without the glass. Look straight past the leading camel, low down at the horizon, and now raise your eyes. What can you see?" "Glare," said Frank. "Try again." "Nothing but more glare, and the atmosphere quivering as it rises from the sand." "Try once more," said the professor. "I can see one--two--three. Look higher." "Ah, I've got it now; a mere speck," said Frank eagerly--"a crow." "Make it vulture, and you will be right. I can make out three--four of the loathsome creatures on their way to the feast. They are making a circuit so as to drop down after we have gone by."