Mirages in the Desert
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Gaspard Monge from Louis Reybaud, Histoire de l’expédition française en Égypte (Paris 1830-36) v. 6.

Gaspard Monge, a mathematician, was one of the leading members of the French scientific contingent. Already in his fifties at the time of the Egyptian expedition, Monge was a highly regarded professor, administrator, and theoretician who was known particularly for his work in descriptive geometry.

As a close friend of Napoleon, and as first president of the Institute, he should have busied himself with carrying out Napoleon’s suggested research programs. Instead, Monge became fascinated with the physics of mirages. The phenomenon became known to the French army quite early, when they marched across the desert from Alexandria to Cairo. The soldiers saw what seemed to be immense lakes in the distance. A distant village might appear as if it were an island, surrounded by a lake with the inverted image of the village reflecting off its surface.

Exhausted by heat and fatigue, they saw the inviting and large body of water retreat as they gradually approached until it finally disappeared altogether, to their great disappointment. The same vision would then repeat itself further on, again and again. Knowledge of the mirage effect was not new, as there are numerous reports and descriptions in the scientific literature and in the writings of antiquity, such as that of Diodorus. No one before Monge, however, had tried to give an optical explanation for mirages.

Pyramids and sphinx at Giza from Description de l’Égypte Antiquités, v. 5.	A mirage in the desert, from Camille Flammarion, L'Atmosphere (Paris, 1873).

Monge’s interpretation was that light rays were bent by the layer of superheated air just above the sand. Heat radiating from the desert’s surface warms the air immediately above it and reduces the air density. The boundary between this heated layer of air and the layer of cooler, denser air above it acts as a lens, bending or refracting the light rays from the sky back upward toward the cooler layer and thus toward the eyes of the observer. The mirage image seems to come along a straight path that originates from the ground, giving the appearance of a layer of water at the surface. The greater the temperature difference, the greater will be the effect of the mirage.

Monge’s paper explaining the phenomenon may be the most famous memoir contributed to the Institute of Egypt. It was published in the Mémoires sur l’Égypte in 1800, which gave it fairly wide exposure. Seventy years later it was still being cited by the popular scientific writer Camille Flammarion who wrote, perhaps with a bit of national pride, that it was Monge who had succeeded in giving an explanation to the mirages that had surprised and puzzled all the other observers.

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