  We have obtained a 12.5 ksec image of the Hercules Cluster, A2151, with the
{\it ROSAT} PSPC. Comparison of the optical and X-ray data suggest the presence
of at least three distinct subclusters in A2151. The brightest X-ray emission
coincides with the highest-density peak in the galaxy distribution, and is
bimodal. The northern subclump, distinct in position and velocity, has {\it no}
detectable X-ray gas. The eastern subclump, apparent in the optical contour
map, is indistinguishable from the main clump in velocity space but is clearly
visible in the X-ray image. X-ray spectra derived from the central peak of
emission yield a best-fit temperature of 1.6 keV. The emission coincident with
the eastern clump of galaxies is cooler, 0.8 keV, and is outside the 90\%
confidence intervals of the central peak temperature. We suggest that the
eastern and central subclusters have recently undergone a merger event. The
lack of X-ray emission to the north suggests that those galaxies do not form a
physically-distinct structure (i.e. they are not located within a distinct
gravitational potential), but rather that they are falling into the cluster
core along the filament defined by the Hercules Supercluster.

Astrophysics