  A general identification of the {\em positional specific heat} as the
thermodynamic response function associated with the {\em static relaxation
length} is proposed, and a phenomenological description for the thermal
dependence of the static relaxation length in supercooled liquids is presented.
Accordingly, through a phenomenological determination of positional specific
heat of supercooled liquids, we arrive at the thermal variation of the static
relaxation length $\xi$, which is found to vary in accordance with $\xi \sim
(T-T_0)^{-\nu}$ in the quasi-equilibrium supercooled temperature regime, where
$T_0$ is the Vogel-Fulcher temperature and exponent $\nu$ equals unity. This
result to a certain degree agrees with that obtained from mean field theory of
random-first-order transition, which suggests a power law temperature variation
for $\xi$ with an apparent divergence at $T_0$. However, the phenomenological
exponent $\nu = 1$, is higher than the corresponding mean field estimate
(becoming exact in infinite dimensions), and in perfect agreement with the
relaxation length exponent as obtained from the numerical simulations of the
same models of structural glass in three spatial dimensions.
