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fore lie in that line or its prolongation. Assume, provisionally, that is the instantaneous centre, and suppose the line c d to be fixed, so that both a and b may revolve as they did at first, then the two centroids, both moving, roll upon one another in 0, and have therefore at that point the
Fig. 22.
same peripheral velocities. Their angular velocities will have to each other the ratio d: cO. But by hypothesis this ratio is constant, there- fore also d and c themselves must be constant; and the centroids become circles described about c and d with radii which are in inverse
