As they approached the Nile, Frank and Fred vied with each other to get the first glimpse of the great river. The mirage was all around them, and the boys were several times deceived by it, until Abdul came to their assistance. "I will give you a rule," said he, "by which you can always tell a real lake or river from an imaginary one. In a mirage the imaginary lake is of the same color as the sky above it, while the Nile is of a deeper blue. You will rarely, if ever, find the sky and water of the same hue when the sun is shining, and this is the only time when the mirage appears. With this simple rule in mind, you are not likely to be deceived." [Illustration: MOUNTAIN PASS IN THE DESERT.] While they were talking Fred made good use of his eyes, and suddenly pointed ahead to a little cleft in the line of rocky hills. There was a strip of blue which did not resemble the sky in color, and he felt certain it was the Nile. "You are right," said Abdul, "and that is the Nile. We will make our camp to-night on its banks." Soon a fringe of palm-trees came into sight; the blue streak increased to a broad expanse of water, and the line of palms into a grove. Then they came among fields of beans and other green things, and before sunset they stood on the bank of the river, and drank freely of the water, which Ali brought them in a large bowl. It was a great improvement upon the warm and strongly flavored water which had been their only resource for eight long days, and they both declared it the sweetest draught they had ever taken. "You can understand now, better than ever before," said the Doctor, "why the Arabs seem to worship the Nile, and why the ancient Egyptians regarded it as a divinity. Without it all this part of Africa would be like the desert we have just passed, and existence here would be impossible." "And I understand, too," responded Fred, "why the Arab conception of Paradise abounds in rivers of never-failing water. Mohammed wrote from his own experience, as he lived among the deserts of Arabia." An Arab merchant, with whom Abdul was acquainted, came to offer the shelter of his house to the strangers; but they preferred their tents, and told the dragoman to thank him for his offer and decline it. Frank proposed a swim in the river, which was seconded by Fred. Abdul suggested the possibility of an interview with a crocodile, and the swim was indefinitely postponed. Crocodiles are numerous in this part of the river, and in fact all the way from the second Cataract to the equatorial lakes. They are by no means timid, and the stranger should think twice before venturing into the river.