  We use an analogy with the statistical mechanics of gas to build the
statistical mechanics of granular media. The case of an isotropic disordered
packing of equal spheres submitted to an isotropic stress is considered. We use
the assumption of a maximal disorder and the method of the Lagrange multipliers
to demonstrate that the distribution of the force moduli decreases
exponentially and obeys an exponential-decay statistics. This distribution
results from the above hypotheses and from the existence of a mean stress,
which acts as a constraint. This distribution is confirmed by experiments and
simulations. We apply the same method to find the distribution of void defects
of a granular assembly. It follows also an exponential-decay statistics.
