The most important event in Egypt during the reign of Mutawakkil was the falling in of the Nilometer at Fostat.
"History Of Egypt From 330 B.C. To The Present Time, Volume 11 (of 12)" by S. Rappoport
Such a column was called a Nilometer.
"Xerxes" by Jacob Abbott
Not far from the spot mentioned above is the famous Nilometer that Moses looked upon many a time.
"Birdseye Views of Far Lands" by James T. Nichols
This Nilometer is an ancient octagon pillar of red stone in the island of Rhoda, on the sides of which graduated scales are engraved.
"Visits To Monasteries in the Levant" by Robert Curzon
From Old Cairo we crossed over to the Island of Roda, to see the Nilometer.
"What We Saw in Egypt" by Anonymous
This is the isle of Rodda, on the bank of which Moses was found, and where you may see the Nilometer.
"The Rulers of the Mediterranean" by Richard Harding Davis