Mass Transport ofHe4Films Adsorbed on Graphite

Flow of ${\mathrm{He}}^4$ films adsorbed on graphite is qualitatively different from flow on other substrates. We observe two distinct types of "onset" phenomena. Type $A$ appears for films thicker than 2 layers at temperatures $T_A$ within 20 mK of $T_{\lambda }$. In thicker films there is an additional more dramatic increase (onset $B$) at lower $T$. $T_B$ varies with $\frac{P}{P_0}$, and is higher than temperatures of "superfluid onset" reported previously.