A high-speed framing camera has been used to photograph deuterium and helium discharges in which flute instabilities are driven by the inertial force of a radial acceleration. The wavelength of the instability increases with decreasing initial pressure, suggesting the damping effect of viscosity; in some cases this damping apparently prevents the growth of flutes entirely. A simple theory of viscous damping is in qualitative agreement with the observations. An instability due to the radial acceleration accompanying a rotation of the plasma appears at about peak field.