  We introduce a new matrix model representation for the generating function of
simple Hurwitz numbers. We calculate the spectral curve of the model and the
associated symplectic invariants developed in [Eynard-Orantin]. As an
application, we prove the conjecture proposed by Bouchard and Marino, relating
Hurwitz numbers to the spectral invariants of the Lambert curve exp(x)=y
exp(-y).
