  We describe a series of surface levelling experiments in glassy polystyrene
(PS) of varying molecular weight. The evolution through a mobile surface layer
is described by the glassy thin film equation that was introduced and used in a
previous work. Excellent agreement with the data is achieved, with surface
mobility as the single free parameter. Different molecular-weight dependencies
in mobility are then observed above and below the glass transition. The results
are discussed in terms of surface-chain anchoring in the bulk matrix, and the
length scale for surface mobility.
