1 The Birth of Science - MSRI

104 4. Scientific Technology
about modern lighthouses. It is enough to know that a part of the ancient
theory of conic sections, comprising the focal property of parabolas, was
recovered in the first half of the seventeenth century (chiefly by Bonaventura
Cavalieri 85 ), and to make allowances for the time needed to overcome
the technical problems attending the practical construction of lighthouses.
4.6 Hydraulic and Pneumatic Engineering
In the area of water engineering, the full extent of the practical relevance
of Hellenistic scientific knowledge can hardly be overlooked. 86 Hellenistic
aqueducts are remarkably well-documented by archeology. Remnants of
water supply systems have been found at several sites, although many
technical features are not yet well understood and in some cases even a
Hellenistic dating has been challenged.
One of the main characteristics of Hellenistic aqueducts is the frequent
use of pressure pipes, which overcame depressions of the terrain thanks to
the principle of the inverted syphon (such pressure pipes are simply called
“syphons” in the archeological literature). For a while the use of syphons
was denied or regarded as exceptional, but they occur in at least seven
of the nine aqueducts known for sure to be Hellenistic. 87 The relationship page 144
between the idea of the inverted syphon and the science of hydrostatics is
obvious. The most remarkable syphon was at Pergamum; it pushed water
uphill to a height of perhaps 190 meters from the deepest point, and the
pressure at the bottom must have been almost 20 atmospheres. 88 It should
be noted that we only know the technological level of these waterworks
because of twentieth-century archeological finds.
85
In Lo specchio ustorio overo trattato delle settioni coniche. . . , Bologna, 1632. Chapter 32 is titled
“How the aforementioned mirrors [the burning mirrors of Archimedes] can be used to shine a
light beam far away at night.”
86
For information on ancient hydraulic engineering, see [Bonnin], [Tölle-Kastenbein], [Hodge:
RAWS], [Wikander: HAWT].
87
[Lewis: HP], p. 646. A much longer list of Hellenistic aqueducts with syphons can be found in
[Hodge: A], p. 43, but for some of them a Roman dating has been proposed.
88
See [Garbrecht]. Hellenistic engineers did not have pressure gauges and so could not measure
the pressure at the bottom. But by the principle of Archimedes (note 65 on page 70), they could
certainly calculate the pressure in the static case: if a U-shaped tube is full of water and in equilibrium,
the pressure at the bottom is that of a column of water as tall as the difference in height
between the bottom and the highest water level (which is the same on both sides because of the
principle of communicating vessels). In an inverted syphon the downhill leg starts higher than the
uphill leg ends and the pressure at the bottom corresponds to a height somewhere in between, as
can be seen from considerations that could hardly have escaped a reader of Archimedes. Thus the
engineers of the time could bracket the pressure within an upper and a lower bound, which in the
case of Pergamum would differ by about 15%.
Revision: 1.14 Date: 2002/10/24 04:25:47