Every child who has read stories of the desert, knows that sometimes travellers over the burning sands often think they see, in the distance, one of those green places with water and palm trees that are called oases. They urge their camels forward only to see the vision fade before their eyes. The animals are not deceived. When near a real oasis they smell the water and hurry of their own accord. Such an unreal appearance is called a mirage. But how is it made? And why does it disappear?

It helps in understanding a great many things to know the meaning of their names. Mirage comes from the same old Latin words from which we get both mirror, a looking-glass, and miracle, a wonder and a mystery. So a mirage is a reflection of some unseen but very real object, sent back from an upper layer of air that, in a way that was long a mystery, acts as a looking-glass.

Many things in nature readily become mirrors. A sheet of quiet water reflects an inverted image of all the things along its banks.

Polished metals and woods, painted surfaces, even glossy leaves reflect. The moon is quite dead and cold. It has no light of its own, but shines by the reflected light of the sun. Even a layer of air may become a mirror. This is the way in which it does so over the desert. As a rule, the air nearest the surface of the earth is heaviest, and light rays from the earth pass upwards through gradually lightening air without being stopped. Over hot, sandy deserts the layer of air on the earth becomes more heated and therefore lighter than air that is some distance above. Light rays are stopped by this denser air layer and reflected back. And just as a tree growing on the bank of a pond is reflected upside down in the water, so a reflection, or mirage, is often inverted in the sky. But sometimes it seems to be lifted and merely reversed as your face is in a mirror. Mirages occur over dry plains and also over the far northern oceans. One case is on record in which the entire north coast of France was reflected to a sea-coast town in England, a distance of fifty miles across the English channel. So you may readily believe that the mirage in the desert is the reflection of an oasis that may be a day's journey distant.