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

Abstract

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).