The Nile is the Gift of Osiris, but Egypt is the gift of the Nile.
Now the Nile, when it overflows, floods not only the Delta, but also the tracts of country on both sides of the stream which are thought to belong to Libya and Arabia, in some places reaching to the extent of two days' journey from its banks.
At the southernmost tip of Cairo's Al-Rawda (Roda) Island where, according to tradition, the biblical baby Moses (the Qur'an's Musa) was found among the bulrushes, there exists one of the oldest structures in Cairo - the Nilometer.
As an instrument of hydrological survey, it is not unique; the various rulers of Egypt have for around five millennia been measuring the River Nile's inundation in order to forecast agricultural yield and to set levies upon its people accordingly. However, the Al-Rawda Nilometer is irreplaceable, both as an example of engineering heritage and as an unparalleled record of Cairean river levels in the Nile since it was (re)constructed in 860 - 861 AD.
The Al-Rawda Nilometer comprises a large stilling well, lined with stone blocks. It is around 10m deep, tapering from about 3m in diameter at the invert to some 5m2 at the top, the walls of which are inscribed with Kufic Qur'anic inscriptions referring to water, prosperity and vegetation. An octagonal white marble column stands vertically at the centre of the well, graduated into 19 cubits1; it is topped with an inscribed wooden2 beam that spans the well at its rim.
Although now dry, once upon a time the well would have been wet, flooded by the Nile waters via gothic-style 'tiers-point' arches open to the river at three levels. The extent of the inundation would be accurately measured against the central column. At low water, the level in the well would have measured around 7 cubits. During the inundation, a measurement of anything between 14 and 16 cubits would have been greeted with much gratefulness to the god of the underworld, Osiris. An inundation below 12 and 13 cubits would have portended starvation and suffering while an inundation exceeding 19 cubits would have meant catastrophic flooding.
However, the taming of the Nile (most significantly by the upstream Aswan Dam) and sealing of the tunnels means that the Nilometer is now redundant as an instrument of contemporary hydrological survey, although it remains significant both as a record of past inundations and as an oft-overlooked attraction for tourists.
715 AD - The Al-Rawda Nilometer was first constructed by the Umayyads under the edict of Suayman Abd al-Malek.
815 AD - Restoration work carried out by Caliph al-Marmoun.
850 AD - Al-Rawda timeline destoyed by flood.
861 AD - Nilometer as we see it today reconstructed under the direction of Ahmad ibn Muhammad al-Hasib by order of the Abbasid Caliph al-Mutawakkil to the design of Abu'l 'Abbas Ahmad ibn Muhammad ibn Kathir al-Farqhani, a native of Farghana, West Turkestan (better known in the West as the astronomer Alfraganus).
872 / 873 AD - Restoration work ordered by Ibn Tulun.
1092 AD - Restoration work ordered by Fatimid Caliph al-Mustansir.
1825 AD - Roof destroyed by nearby factory explosion and reconstructed using 18th Century painting by Danish traveller Fredrik Ludvig Norden.
Today, the Al-Rawda Nilometer resides inside the blissfully peaceful gardened compound of Cairo's Al Manasterly Palace, home to the Cultural Development Fund's International Music Centre - a venue for concerts by internationally-acclaimed artists, prominent Egyptian musicians and promising young talent.
Apart from the appeal of being a sanctuary from the general hullabaloo that so typifies Cairo, the Nilometer has off-the-beaten-track tourism appeal and should be a must-see for engineers, architects and other constructaphiles and their hard-done-by families.
To get there, once you're in Cairo take a taxi or ramble to the southern (upstream) end of Al-Rawda Island. At the time of writing, the entry fee is LE6 (six Egyptian pounds, equating to about 60p).