'Egypt is the Nile' and since the height of the annual inundation of the river was crucial to the economy forecasting it has always been a matter of great interest. The pharonic nilometer was a little upstream at Helwan: this was replaced in 715 by a nilometer on the present site, which in turn was rebuilt in 861 by the mathematician Ahmed ibn Muhammed.
The central pit is connected to the river by three tunnels, and the level of inundation can be measured on the graduated central shaft. And as soon as the level of flooding was known, the government could set the tax rate for the coming year.
The kiosk over the nilometer is Ottoman and has recently been lavishly restored.
The pointed arches in the walls are the earliest known in Egypt, and the carved Kufic script is also a very early example of this kind of script.
The nilometer is in a small park also containing a museum dedicated to Umm Kulthoum, which is the main attraction as far as the Egyptians are concerned. You have to hang around and wait for one of the attendants to unlock the Nilometer: he'll then show you around. The man who showed me round was most informative and thoroughly deserved his baksheesh.
Located at the tip of Roda Island, the Nilometer is very interesteing. It was used to measure the level of the Nile and thus predict upcoming harvests etc.
Cost is 6 pounds, you need to find the attendant to let you in.