Dictionary of islamic architecture

Nilometer
Device located on Roda island, Egypt for measuring the
rise of the Nile during the period of inundation.
This structure was built in 861–2 CE during the reign
of Caliph al-Mutawakil. The purpose of the device
is to measure the level of the flood to work out the
amount of tax due to the government (a higher flood
level indicates a higher yield). The structure consists
of a ·2 m square stone-lined pit 13.14 m deep
connected to the Nile by three tunnels. In the centre
of the square pit is a tall octagonal column divided
into cubits each of which is subdivided into twenty-
four smaller units. A staircase runs down the four
sides so that the central column could be read. The
floor at the bottom was made of cedar beams.
Approximately halfway up there are four pointed
relieving arches, one on each side of the pit. The
arches are of a two-centred type used in Gothic
architecture in Europe during the fourteenth century.
The curves of the arches are emphasized by two
bands of moulding whilst above there are foliated
kufic inscriptions which contain Quranic passages
referring to crops and harvests.
Further reading:
K.A.C.Creswell, 
A Short Account of Early Muslim
Architecture,
revised and enlarged edn. J.W.Allan,
Aldershot 1989, 383–5.
nine-domed mosque
This is a type of mosque roofed by nine domes of equal
size.
Although the distribution of this building type is
very wide (it is found as far apart as East Africa,
Bangladesh, Central Asia and North Africa) it does
not occur in great numbers in any one area. The
earliest extant examples date from the ninth century
CE, whilst there are few buildings of this type later
than the sixteenth century.
Most nine-domed mosques are fairly small
(usually 10–15 m square) though substantially built.
It is common for these buildings to be open on two,
three or even four sides but it is rare for them to have
a sahn or minaret. Sometimes the central row of domes
is raised to emphasize the mihrab axis.
There are two theories about the origin of this type
of mosque. The older theory originated by Creswell
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asserts that the mosque is derived from the earliest
forms of Islamic funerary monuments, such as the
Qubbat al-Sulaybiyya at Samarra, which were
domed and had open sides. More recently it has been
Nine-domed mosque, Kilwa (after Garlake)
nine-domed mosque

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suggested that the type derives from honorific
buildings such as Khirbet al-Mafjar. Whilst the origin
of the design may be disputed it is clear that many
of the mosques were associated with tombs or burials
so that it is reasonable to suggest that they should
be regarded as memorial mosques. Important
examples can be found at Kilwa, Fustat, and Toledo.