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164                                    Science
specially used 'when the dekan stars are not visible*. The oldest
water clock dates from about 1300 B.C.   An account of one
specimen, of the same type, in an Eighteenth-Dynasty tomb is
of special interest as the earliest record of physical observations.
It gives a relation between the lengths of the summer and winter
The water clock, later called by the Greeks the clepsydra, took
the form of a vessel shaped like a flower-pot. In use, it was
Tilled to a certain mark with water, which flowed out gradually
through a small aperture near the base. On the interior surface
a series of marks corresponding to the water-level at the various
hours of the night indicated the 'hours'. These 'hours' were not
of uniform length, but in the absence of regularly moving
mechanism or precise methods of observing the movements of
the stars, the irregularities were probably not noticed. Clep-
sydrae were used in temples to enable the hours of duty of the
attendants to be apportioned. The 'hour*, being thought of as
the twelfth part of the night, varied according to the season of
the year, and a scale of markings for each month was pro-
Some unusual features of the only inflow type of water clock
ertant (c. 100 B.C.) seem to point to the Egyptian origin of
several passages in classical literature dealing with the rate of
increase in the length of the day from winter to summer. In
this type of water clock, water was allowed to drip in from a
reservoir, and time intervals were gauged by the rise of the
water-level against a scale of markings on the interior surface—
the Nilometer in miniature.
The hours of the day were very roughly determined by simple
shadow clocks, evolved independently of the clepsydra. Shadow
clocks are still in use in country districts and to this day the
'servant gapeth after the shadow' (Job vii. 2 margin), when it is
nearing time to knock off work. In the East, except in towns,
there is still little need for mean solar time, and clocks and