to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

When gas and liquid flow in a pipe the interface between the phases may take a variety ofdifferent patterns, the most fascinating and misunderstood being probably slug flow. Asthis kind of flow occurs over a very wide range of intermediate flow rates of gas andliquid, it is of interest for many practical problems involving flow instabilities and transientbehaviour.

Ura Cnrs

Papers

V oid and pressure w aves in slug f low