In the center of the pit a marble, octagonal column with a Corinthian capital that rises from its depths surmounting a millstone. At the top there is a wooden beam spanning the Nilometer. To measure the Nile flood, this column is graded and divided into 19 cubits (a cubit is slightly more than half a meter, and hence, it was capable of measuring floods up to about 9.2 meters). The flood that this Nileometer measured was both important to the rulers of Egypt as well as the whole population. An ideal flood filed the Nilometer up to the sixteenth mark and Looking down the column of the Nilometer with Kufic text visible at the top of the picture less then this could mean drought and famine. On the other hand, if the measurement exceed the 19 cubits, a catastrophic flood was at hand. In the days prior to the expected flood, this column would be anointed with saffron and musk in order to help induce a good water level.
Plain Kufic inscriptions adorn the walls of the Nilometer and are the earliest surviving examples of architectural epigraphy (inscriptions considered as a group) in Egypt. They are taken from Quranic texts that refers to water, vegetation and prosperity, and therefore have a talismanic meaning, but there is also secular text as well. These inscriptions were executed in white marble originally on a blue background, though the letters themselves were left in the natural stone color. However, the inscription recording the establishment of the Nileomter by al-Mutawakkil has been removed. Creswell, a well known historian of this period, believes this was done by Ibn Tulun, who replaced it with additional verse, as part of a campaign to assert his independence from the foreign Caliphate. Part of the original inscriptions read:
"We send down rain as a blessing from heaven, whereby we cause gardens to spring forth and the grain to harvest." (50:9) "Hast thou not seen how that God has sent down out of heaven water, and in the morning the earth becomes green?" (22:62)
Because of its importance in determining the prosperity Egypt would experience during the following year, this Nilometer was the departure point of the greatest of Cairo's celebrations throughout the medieval period. This was the Fath al-Khalij, the festival of the Opening of the Another recent exterior view of the Nilometer on Rawda Island Canal, which ceased in 1899 when the Khalij (Khalig) was filled in (this Nilometer itself probably continued to be used up until the last flood in 1970). The Khalij Canal started opposite Rawda Island, bordered the medieval city to the west, and irrigated its outlying gardens and fields. The Khalij canal was blocked with an earth dam and was cleaned before the flood. It would then be opened when the water level reached the sixteenth cubit level, when the caliph and later sultans and pashas would inaugurate the celebrations that lasted for several days. The summer flood from the Nile would then fill this canal, together with many ponds that would have winter beds green with vegetation. During the celebrations, decorated boats would crowed the waters, and among these, the most splendid would be that of the ruler. Those who witnessed this event refer to it as Cairo's most spectacular celebration. During the hot summer months, the Khalij and the ponds remained filled with pleasure boats and its shores were lined with entertainment. Near the Nilometer was a mosque for prayers during the flood celebration, and a palace for banquets held by the various rulers.
However, the grand celebration was not guaranteed as an annual event. When the Nile flood waters failed to reach the sixteenth cubic mark, the celebrations were canceled and prayers and fasting were held instead in order to ward off the expected drought and famine.
http://www.touregypt.net/featurestories ... erroda.htm
thanks for that Ebi 
