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\item Why does the hydrodynamic disturbance travel diffusively rather than at the sound speed?  \item Some reading about propagation of hydrodynamic information:\\  \citet{10.1017/S0022112075003102} application of Langevin equation to fluid suspensions.\\  \citet{Ladd_1993} short-time motion of colloidal particles: theory.\\  \citet{1995PhyA..214..185E},  \citet{Espanol_Rubio_Zuniga_1995} for discussion of how HD disturbances propagate, and evidence for faster-than-vorticity interaction. One considers incompressible and the other considers compressible; the vorticity time-scale is not the only important time-scale.\\