  We derive isospectral flows of the mass density in the string boundary value
problem corresponding to general boundary conditions. In particular, we show
that certain class of rational flows produces in a suitable limit all flows
generated by polynomials in negative powers of the spectral parameter. We
illustrate the theory with concrete examples of isospectral flows of discrete
mass densities which we prove to be Hamiltonian and for which we provide
explicit solutions of equations of motion in terms of Stieltjes continued
fractions and Hankel determinants.
