Let $L$ be a time-periodic Tonelli Lagrangian on a closed manifold of
dimension two. Then the $\beta$-function of $L$ is differentiable in at least
$k$ directions at any $k$-irrational homology class. The same result holds when
$L$ is an autonomous mechanical Lagrangian with a $C^3$ potential on a closed
manifold of dimension three.
We prove that Ma\~n\'e's conjecture, as stated in {\em Lagrangian flows: the
dynamics of globally minimizing orbits}, Bol. Soc. Brasil. Mat. (N.S.) 28
(1997), no. 2, 141--153, contains another conjecture of Ma\~n\'e, stated in
{\em Generic properties and problems of minimizing measures of Lagrangian
systems} Nonlinearity 9 (1996) 273-310.