A Precedent

Lanchester’s critics insisted on reading his work as an exercise in inviscid fluid theory. Bryan, Taylor, Bairstow, Cowley, and Levy all made this move. This should be puzzling. It must have been evident to any reader of Lanchester’s book that he was not simply thinking in terms of perfect fluids. He was con­stantly moving back and forth between viscous and inviscid approaches, try­ing, as it were, to negotiate some rapprochement between them. How could this have been overlooked? One answer is that it was not overlooked at all. Perhaps it was perceived clearly but seen as a weakness in the text. The inclu­sion of both viscous and inviscid strands in the argument may have seemed like mere ambiguity. Reading Lanchester as a purely inviscid theorist may have been a way to repair the ambiguity. Wouldn’t this be a natural thing for mathematically sophisticated readers to do?

There may be some truth in this suggestion, but it cannot be the whole story. It does not explain why the ambiguity was resolved by turning Lan – chester into an exponent of the inviscid approach rather than an exponent of a viscous approach. Either would have resolved the ambiguity, so why did the inviscid reading prevail? Here is a possible answer to that question. There was a precedent for the preferred assimilation, namely, the received understanding of Rayleigh’s paper on the irregular flight of the tennis ball. Rayleigh had done exactly what Lanchester had done, that is, work with an informal mixture of ideal-fluid theory and viscous considerations. The math­ematics of Rayleigh’s tennis ball paper dealt with an inviscid fluid, but the circulation described by this mathematics could not have arisen from any processes conceptualized within it. Friction was needed to account for the circulation. The fluid was therefore taken to be viscous at one point in the account and inviscid at another point, thus rendering the argument logically inconsistent. How did readers respond to this oscillation between viscous and inviscid fluids? In Cambridge the tennis ball paper was absorbed into the literature on inviscid theory. The ambiguity was resolved by playing down the appeal to viscosity and treating the analysis as if it began at the point where a circulation could be taken as a given. This approach had the virtue of focusing on the part of the work that was easiest to develop mathematically, and indeed it may explain why the assimilation went this way rather than the other. It was in these terms that Rayleigh’s tennis ball result found its way into the Cambridge examination papers.6