  We compute the entropy of entanglement of two blocks of L spins at a distance
d in the ground state of an Ising chain in an external transverse magnetic
field. We numerically study the von Neumann entropy for different values of the
transverse field. At the critical point we obtain analytical results for blocks
of size L=1 and L=2. In the general case, the critical entropy is shown to be
additive when d goes to infinity. Finally, based on simple arguments, we derive
an expression for the entropy at the critical point as a function of both L and
d. This formula is in excellent agreement with numerical results.
