  We show how the rotation and translation fields of a surface, introduced by
G. Darboux, may be used to obtain short proofs of a well-known theorem (that
reads that the total mean curvature of a surface is stationary under an
infinitesimal bending) and a new theorem (that reads that every infinitesimal
flex of any simply connected closed surface is orthogonal to the surface at
least at two points).
