Archaeology Magazine - July/August 1986 - 65

(though his own description implies that
he derived ideas from much older literature) and its hydrodynamic concept is
the one that, after some ingenious adaptations by the Greeks, dominated the
way in which water clocks were designed, and time was recorded, for the
subsequent three millennia.
Early improvements
How well did the first clepsydra work?
The shape of the vessel- in essence, the
inverted frustrum of a cone- shows that
Amenemhet was aware of the fact that
the rate of water discharge decreased as
the water level fell. By tapering the vessel, he ensured that, as the discharge
slowed, the volume of water given up
during each time increment decreased
in a compensatory manner. He also took
account of the differences in seasonal
night-length by including a longer scale
for use in the winter months.
Despite these features, however, it is
unlikely that the accuracy of hourly recording maintained by each clepsydra
was better than several minutes either
way. We know now that the fluid mechanics of an outflow system of this kind
are sensitive to several other parameters besides the water head's height.
Most obviously, variations in the bore
of the outlet pipe (presumably a reed)
of only fractions of a millimeter, and/or
the sharp fluctuations in daily and seasonal temperatures of as much as 18°C
that characterize the Egyptian climate,
would have conspired to make the clock's
working quite uneven.
The Amenemhet unit was soon expanded to comprise two vessels, with
an old-style clepsydra acting as a supply reservoir for a graduated receiver
placed beneath its outflow tip. (I suspect this was a domestic variant in
which the water could be recycled each

day: certainly it offered no improvement
in timekeeping accuracy. ) Then, around
250 b.c., the Greek scientist Ctesibios
redesigned the entire thing. He added
an overflow pipe at the rim of the outflow clepsydra so that it became a true
reservoir fed by a far larger vessel (or a
stream) which kept the head of water
fully topped up. A float-mounted vertical rod was added to the receiver.
Ctesibios was not content to let this
rod's tip serve as a pointer for an hourlygraduated scale. In one system he added
teeth to the rising rod which could engage a cog-like drum. This, in turn,
powered an automaton that hurled out
pebbles or eggs every hour. In another
system, he topped the rod with a figure
which pointed at a drum with a series of
graduated scales on its side, each intended to approximate the hours of daylight for a specific month.
Clepsydra design took several new directions by the first century B.c., including a clever modification of the
outflow system of the now metal upper
reservoir, to allow adjustment for seasonal variation in daylight extent. The
outflow pipe was replaced by a rotatable bronze disk with a hole drilled near
its rim. A fixed ring was mounted around
this disk, and marked with the zodiacal
signs: Cancer at the top, Capricorn at
the bottom. With the outflow hole
"dialed" to its highest point, the rate of
water discharge was minimized to match
the longer summer days. It was rotated
to a lower setting each month, so that
the slight increase in the head of water
(as measured from the disk's hole to the
reservoir's overflow level) appropriately
raised the discharge rate. This and a
host of other novel approaches to the
construction of the float system meant
that the water clock became that bit
more precise a time-piece, and therefore
of more universal value.
from Archimedes
Inspiration
But I have moved this story forward
too quickly. At much the same time that
Ctesibios was promoting his new clepsydra design, a Greek mathematicianone of great renown today- devised a
mechanism which opened up a whole
range of ways of using water power to
drive time-marking automata. I speak,
of course, of Archimedes.
Born in about 287 b.c., in the Sicilian
port of Syracuse, he is best known for
the incident where he ran through the
streets shouting "heureka"- apparently
in sheer excitement over solution of the
problem of assaying the gold in the
crown of King Hieron- and for the invention of a helix-shaped pipe (thereafter known as the Archimedean screw)
which provided a convenient and efficient means of raising water. Engineers
perhaps remember him better for his development of the compound pulley, and

of the principle of the lever that prompted
the remark, "Give me a place to stand
on and I will move the Earth."
To the water clock, Archimedes
brought a brilliant modification of the
float chamber- a «imple conical valveplug which was attached to the top of
the float so that it fitted snugly into the
end of the pipe feeding water out of the
upper reservoir. Its operation was almost
oscillatory. As some water flowed out of
the exit pipe of the float chamber, overpressure at the valve plug alloweda brief
water replenishment from the reservoir.
The new water quickly forced the float
upward again and pressed the valve-plug
back into the pipe above it. With the
height of the water column in the float
chamber exerting a constant pressure
head, the rate of outflow of the entire
device was near constant. That then had
to be also true of the rate of fall of the
water level in the reservoir's cylindrical
chamber. It was this water fall that
Archimedes used to power a piniondrum and gear mechanism above it, via
which metal balls were ejected from a
chute every hour during a twelve-hour
cycle.
Exactly when and how this concept
was picked up and expanded upon by
contemporary or later scientists is obscure. The only extant source of the nature of the mechanism is an Arabic
manuscript preserved in various posttwelfth-century forms and called The
Pseudo-Archimedes Treatise, very much
in recognition of the fact that so much of
its content obviously describes automaton designs in which the Greek scholar
had no hand.
A consensus of commentators believe
that a mechanics-minded philosopher,
Philo of Byzantium, contributed at least
two sections to that treatise. He visited
Alexandria around 250 b.c., and may
well have met Archimedes there. Philo
tapped the uniform speed of rotation of
Archimedes' pinion-drum component to
drive sets of counterweighted pulleys.
These in turn powered various kinds of
time-measuring units, e.g., a ring collar
mounted over a graduated column.
Islamic elaborations
These ideas of Ctesibios, Archimedes
and Philo were part and parcel of the
gamut of Greek original thought which
was avidly sought and absorbed by the
Islamic scientific community a millennium later (see "ScienceScope,"Archaeology, July/August 1985). In the early
ninth century A.D., the brothers Banu
Musa incorporated several of the hydrostatic principles underlying the making
of water clocks in the construction of
all manner of trick jars and lamps that
kept the khalif's court entertained for
hours. By the early thirteenth century,
the most imaginative of all Islamic clockmakers, Ibn al-Razzaz al-Jazari, could
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1986 65
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Archaeology Magazine - July/August 1986

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