It is shown that, for vortex trails in a nonviscous fluid, themeanlongitudinal spacingaandmeantransverse spacinghare invariant. It follows from this and other known facts, that theperiodicityof a vortex trail is unstable, not the ratioh/a.In a viscous fluid,ais invariant buthincreases. This fact explains various known experimental observations.A rough explanation is given for the order of magnitude of the Strouhal numberS, based on the inertia opposed by the wake to transverse oscillations.Finally, using the theory of logarithmic potential, it is shown that, even in a nonviscous fluid, the concentration of vorticity is limited to circles whose diameter is abouta/4.