  A certain class of surface motions, including those of a relativistic
membrane minimizing the 3-dimensional volume swept out in Minkowski-space, is
shown to be equivalent to 3-dimensional steady-state irrotational inviscid
isentropic gas-dynamics. The SU($\infty $) Nahm equations turn out to
correspond to motions where the time $t$ at which the surface moves through the
point $\ssatop\rhup{r}$ is a harmonic function of the three space-coordinates.
This solution also implies the linearisation of a non-trivial-looking scalar
field theory.
