Not only do the size of the river-beds show that there used to be more frequent rains in these regions of desolation, but right at the edge of the northern Sahara are the remains of immense aqueducts; great troughs built of stone and carried on bridges from the source of a water supply to a city. When the Romans owned the earth--including the Sahara desert--they were famous builders of these aqueducts. [Illustration: WHY DYING RIVERS MULTIPLY BY TWO Director Hornaday, of the New York Zoo, took this picture while in the arid regions of the great Southwest. It shows a little stream dying away in the desert sands. Now just notice how a little knowledge of nature's methods as a landscape artist makes the most commonplace scenery interesting. All streams as they go dry have a tendency to spread out arms like that; sometimes two, sometimes four or more, but always in twos or multiples of two. The reason is that as the water evaporates the stream becomes weaker and so is obliged to drop a part of its load. The heaviest part of the load--the most pebbles, sand, and soil--is carried in the middle of the stream, owing to the current being stronger, relieved as it is from the friction of the banks. So bars of sand, gravel, and such stuff are built up that finally divide the water into two branches. Then if the water keeps on flowing, each of these branches divides by two, and so on. You see the same thing in the mouths of deltas. ] "But what about the roses made of sand? That's a conundrum you didn't answer." Oh, yes, we must get down closer to the desert to see these. We can't see them in the bird's-eye view we have been taking. The desert sand has a great deal of gypsum in it, and when the sand gets a wetting from a cloudburst this gypsum crystallizes and forms what are called "sand roses." These "roses" are of various sizes and forms; some look like camelias and some like a cluster of pearls. They are not common and you have to hunt for them. [Illustration: ALL THE COMFORTS OF HOME Children in the primary grades have here told us, with their clever little fingers, about life in Africa immediately south of the big desert, the part of Africa where they have rain and to spare. ] II. How the Desert Makes Its Sand Most of the sand of the desert, as you may imagine, is home-made; and it is very curious to notice the different which it is manufactured. The desert sun and the cloudless nights have a great deal to do with it. [Illustration: HOW THE ARAB FARMER GATHERS HIS DATES]