1. Other variation principles have also been used: cf. J. Serrin, inHandbuch der Physik, (Springer-Verlag, Berlin, 1959), Bd VII∕1, pp. 145–206.
2. Equation (3.14) could be rewritten in terms of the covariant derivative used in differential geometry. The same remark will apply to Eq. (4.6).
3. This is true only because Q = q is the basic solution. The equations of Sec. III may also be linearized, and are then also derivable from a Lagrangian. However, this is the sum of thefirst and secondorder terms ofL. The linearized equations of motion will therefore be inhomogeneous, in general fluid coordinates.