  We report an experimental investigation of the transition from a hexagonally
ordered solid phase to a disordered liquid in a monolayer of vibrated spheres.
The transition occurs as the intensity of the vibration amplitude is increased.
Measurements of the density of dislocations and the positional and
orientational correlation functions show evidence for a dislocation-mediated
continuous transition from a solid phase with long-range order to a liquid with
only short-range order. The results show a strong similarity to simulations of
melting of hard disks in equilibrium, despite the fact that the granular
monolayer is far from equilibrium due to the effects of interparticle
dissipation and the vibrational forcing.
