  In 3-dimensional hyperbolic geometry, the classical Schlafli formula
expresses the variation of the volume of a hyperbolic polyhedron in terms of
the length of its edges and of the variation of its dihedral angles. We prove a
similar formula for the variation of the volume of the convex core of a
geometrically finite hyperbolic 3--manifold M, as we vary the hyperbolic metric
of M. In this case, the pleating locus of the boundary of the convex core is
not constant any more, but we showed in an earlier paper that the variation of
the bending of the boundary of the convex core is described by a geodesic
lamination with a certain transverse distribution. We prove that the variation
of the volume of the convex core is then equal to 1/2 the length of this
transverse distribution.
