Evidence regarding second-order clustering of galaxies and interactions between clusters of galaxies

The evidence for the existence of second-order clusters is reviewed. A statistical analysis of the distribution of the rich clusters identified on the Palomar Sky Survey, the observed extent of 17 typical apparent groupings of galaxies, and de Vaucouleurs' study of the Local Supercluster all indicate a mean linear diameter of the order of 50X 100 pc for second-order clusters. Typical masses for these systems are estimated at from 1016 to 10' solar masses. If the systems are gravitationally stable, they should have internal root-mean-square velocity dispersions in the range 1-3X 10 km/sec. The possible influence of second-order clusters upon the observations relevant to the stability of systems of galaxies is discussed.