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Response to the comments of D. R. Blackman and Y. Peleg (JRA 14, 411-14)

Published online by Cambridge University Press:  16 February 2015

Paul Kessener
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Extract

The comments on my article (JRA 13,104-32) by Blackman and Peleg, which are much appreciated, reflect prevailing discussions on Roman hydraulic engineering practices as well as on the relevant paragraphs of Vitruvius (8.6). Blackman focuses mainly on technical aspects, while Peleg aims at archaeological arguments.

Blackman starts with a discussion of the term ‘siphon’, which he regrets being used as it could lead to misunderstanding: ‘siphon’ would represent a “real” siphon, not an “inverted” siphon, suggesting that I must have meant that there was a real siphon at Aspendos. However, in archaeology “siphon” is generally accepted to represent an inverted siphon. There is no misunderstanding: at Aspendos we have an inverted siphon, and since real siphons are not known in classical aqueduct systems, there is no objection to using ‘siphon’ and ‘inverted siphon’ for one and the same notion. Subsequently he states: “The presence of air pockets and so on is almost irrelevant to operations [of siphons] if no point in the system lies above the Hydraulic Gradient Line; were it not so, no garden hose would work reliably” This is a misconception. It is only due to the fact that our garden hoses are connected to supplies with elevated pressures that we get water from them. If we would connect the hose to a low-pressure source (e.g., to a rainwater container standing at ground level), we have to straighten out the coils before water will emerge. If the hose is coiled up, for example, on a wall support but positioned below the free water surface in the container to which it is connected, while we are holding the free end somewhere near the ground, we have nothing but an inverted siphon with high points. If air pockets are irrelevant, water should come out, but it does not if air is in the hose. For problems associated with air pockets in gravity-driven closed conduit systems (which classical siphons must be considered to be), see G. Corcos, Air in water pipes (1989) and H. T. Falvey, Air-water flow in hydraulic structures (1980).

Type
Responses
Information
Journal of Roman Archaeology , Volume 15 , 2002 , pp. 349 - 351
Copyright
Copyright © Journal of Roman Archaeology L.L.C. 2002

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References

1 Hodge, A. T., Roman aqueducts and water supply (London 1992) 147 Google Scholar, and n.38: “By ‘siphon’ a classical archaeologist traditionally refers to what an engineer more strictly calls an ‘inverted siphon’ …”

2 Only in one case has a real siphon from Roman times been claimed, the Barratina siphon at Termini Imerese ( Belvedere, O., L'acquedotto Cornelio di Termini Imerese [Rome 1986] 115–27Google Scholar). This siphon presumably carried water to the city by means of a 1300-m lead piping passing over the top of an intermediate tower at the far side of a valley. The top of this tower rose more than 6 m above the Hydraulic Gradient Line. One wonders why the Roman engineers at Termini Imerese would have wanted to include a constructed intermediate high point, rising over 6 m above ground level, when they would have had no problems leaving it out. It seems more likely that the construction is related to the Figurella/ Favara aqueduct which ran nearby.

3 In a conduit sloping down, an air bubble will move with the flow if the velocity exceeds a specific critical value which is related to bubble size etc. The steeper the slope and the larger the bubble size, the less readily will the bubbles move with the flow. In certain conditions large air bubbles (‘slugs’) may move against the flow while small bubbles move with the flow ( Kessener, H. P. M., “Vitruvius and the conveyance of water,” BABesch 76 [2001] n.42Google Scholar).

4 For a calculation of this estimate see Kessener ibid. 154 n.47.

5 Blackman refers to negative pressure waves that would occur if any incident-positive pressure wave is reflected from a large air pocket. However, as the density of the air is less that that of water, it is reflected as a positive, not a negative, pressure wave. This situation is different from the wave in the pond that hits a wall, described in Blackman's n.4: the wall consists of material of higher density than water, and only in that case is a positive wave reflected as a negative one (I am grateful for discussion to G. Bruls of the Department of Physics, Frankfurt University).

6 Blackman adds here a discussion on effects of a change of direction of the internal flow, which for the relatively low flow velocities in ancient siphon systems is small compared to forces from static pressure (see my article 125).

7 Compare the coiled up garden hose above. I can make available calculations for the case of Aspendos.

8 Kessener (supra n.3) 142 and figs. 7-8.

9 For discussion on colliquiaria, see Kessener (supra n.3).

10 Ibid.