Nilometers were used in Egypt particularly during the pharaonic, Roman and medieval periods to measure the Nile’s water level during the annual summer flood.
Essentially, apart from any additional ritual function they had in ancient Egypt, nilometers worked on the Goldilocks principle as far as the rulers of Egypt were concerned. If the water level in August-September was too high, the effects on the narrow band of fields either side of the river, and the surrounding infrastructure, would be devastating and there was no point sending out tax collectors. If the water level was too low, the population would starve, and there was also no point sending out tax collectors. If the water level was just right, a rich layer of alluvial silt would be deposited as the flood subsided, crops would grow, and off would go the tax collectors, no doubt singing merrily to themselves.
The structures were of three types: a simple vertical column in the water (as in the medieval example in central Cairo, the modern ornate housing of which is photographed above); a well with a culvert to the Nile (as at Kom Ombo temple); or a set of steps cut into the bank (as at Elephantine Island, Aswan). Each was calibrated in Egyptian cubits (roughly 54 centimeters or 21 inches) subdivided into seven palms, and then into four fingers.
The medieval nilometer at the end of Roda Island in Cairo—though of the simplest type—is particularly beautiful, with its nineteenth-century conical roof.
Although standing on the site of an older building from 715 A.D. (and probably on an older pharaonic nilometer), the current structure dates to 861, when it was ordered to be built by the Abbasid caliph al-Muttawakil. It was restored by Ibn Tulun in 872-873, and again in 1092 by the Fatimid caliph, al-Mustansir.
Three tunnels at different heights lead in from the river to a stone-lined pit around a central marble column marked in 19 cubits. If the flood measured much below 16 cubits, it was too low, and if it measured above 19 the flood was too high.
Around the top of the pit are Qur’anic inscriptions in Kufic script concerning rain and an abundant harvest.
As noted, the building is mostly original, apart from the conical roof which is based on an earlier example which was destroyed by bombardment during the French occupation. When its replacement was destroyed by a nearby factory explosion in 1825, the current dome was constructed based on an eighteenth-century painting by the Danish traveler, Frederik Ludvig Norden.
After the initial damming of the Nile at Aswan in the early twentieth century, nilometers began to fall gradually out of use, and, indeed, this example no longer connects to the river at all, though it may have done so as late as 1970.