A nilometer was a structure for measuring the Nile River's clarity and water level during the annual flood season. There were three main types of nilometers, calibrated in Egyptian cubits: (1) a vertical column, (2) a corridor stairway of steps leading down to the Nile, or (3) a deep well with culvert. If the water level was low, there would be famine. If it was too high, it would be destructive. There was a specific mark that indicated how high the flood should be if the fields were to get good soil. The simplest nilometer design is a vertical column submerged in the waters of the river, with marked intervals indicating the depth of the water. One that follows this simple design, housed in an elaborate and ornate stone structure, is on the island of Rhoda in central Cairo. This nilometer dates back to 861 AD, when the Abbasid caliph al-Mutawakkil ordered its construction, overseen by the astronomer Afraganus. David Roberts (October 24, 1796 - November 25, 1864) was a Scottish painter. He is especially known for a prolific series of detailed lithograph prints of Egypt and the Near East that he produced from sketches he made during long tours of the region (1838-1840).