  We report an experimental study of 2D dipole clusters formed by small
magnetic particles floating at the air-water interface. The particles form
hexagonally ordered clusters due to interplay between dipole-dipole repulsion
and confining external forces. Optical images of the clusters allow
determination of area S, total energy E, and the chemical potential for
different particle numbers N<130. Asymptotic expressions for E(N), S(N) account
fairly well for the smooth part of our experimental results. However, on top of
the smooth dependences we observe quasiperiodic fluctuations with dips at
``magic'' numbers corresponding to particularly symmetric cluster
configurations. The magnitude and occurrence of these fluctuations are related
to the symmetry of the cluster and to the center of mass position. This is in
full agreement with the recent prediction of Koulakov and Shklovskii for Wigner
crystal island.
