A compact cluster of 3 to 6 rigid equal spheres is falling under gravity in a viscous liquid. The small effects of intertia on a horizontal regular polygonal configuration are that the polygon expands as it falls and small perturbations from this configuration die out, although when the polygon is large enough it becomes weakly unstable. This is an extension of the analysis of Hocking (1964, which was applied to the experiments of Jayaweera, Mason & Slack (1964).