  In this paper, we describe a semi-discrete method for a numerical resolution
of a type of partial differential equations, called the method of lines (MOL).
This method is based on the discretization of all but one of the variables of
the problem. We illustrate this method by solving the Laplace equation in
Cartesian coordinates. We compare the concepts used by the MOL with respect to
the analytical method of variable separation. We show that the results obtained
with the MOL are very good approximations of the analytical solutions.
