1 The Birth of Science - MSRI

90 4. Scientific Technology
sydra: the water timers used in trials and mentioned by Aristotle 17 were
clepsydras, if anything simpler than the Egyptian ones. 18
The first true clocks appeared in Alexandria in the first half of the third
century B.C., thanks to Ctesibius, 19 who transformed the old clepsydra
into a true measuring instrument. We know from Vitruvius that Ctesibius
solved brilliantly all the problems mentioned above. 20 The water reservoir
had two openings, a small one in the bottom and a larger escape hole
on the wall; it was continually refilled at a rate intermediate between the
flow rates of the two openings, so the water remained at the level of the
upper hole, ensuring constant pressure. 21 The lower orifice was drilled in
gold or a precious stone, to avoid corrosion and accretions. The water that
flowed out of the bottom was collected in another container, where a float
moved a pointer by means of gears, allowing the reading of the water
level against a scale. 22 In one model multiple reading scales were set on a
cylinder that rotated so as to display the scale appropriate to the time of
year, thus adjusting for the variable duration of the hour.
Several scientists, including Heron, 23 studied water clocks. A remarkable
design, attributed to Archimedes, is described in an anonymous work
preserved in Arabic. 24 This clock, unlike that of Ctesibius, is not replenished
continuously; the main reservoir is filled but once a day and emptied
at a constant rate, so that its level can be read out by the use of a float
connected to a display by appropriate means. Water descends from the
main reservoir into a second chamber through a pipe that ends in a conical
flare that opens downward; inside this flare a conical float valve fits
snugly. When the water in the lower chamber is not at its highest level,
the float valve lets water in until the level rises enough to close the valve
against the flared tube. Thus the water in the lower chamber remains always
practically at its maximum level, and so flows out of the lower chamber
(through a hole near the bottom) at a constant rate; that is also the rate
17
Aristotle, Atheniensium respublica, lxvii, 2–3.
18
Because their function was merely to set a uniform limit to the duration of each party’s address,
these timers had no intermediate marks, much like the hourglasses that come with many games
today.
19
Compare Section 3.5 above.
20
Vitruvius, De architectura, IX, viii; 2–14.
21
This technical solution suggests that the notion that hydrostatic pressure depends on the height
of liquid (stated in the first postulate of Archimedes’ On floating bodies; cf. page 65) was already
clear to Ctesibius.
22
Like the dioptra described by Heron, the gears built by Ctesibius are considered “premature”
by Price ([Price: Gears], p. 53).
23
A fragment of Heron’s treatise on water clocks can be found in [Heron: OO], vol. 1, p. 456.
24
The work is translated and discussed in [Hill: CWC]. The clock is also described in [Lewis:
TH], pp. 364–366.
Revision: 1.14 Date: 2002/10/24 04:25:47