  We carry out calculations on $M$-changing collisions of NH ($^3\Sigma^-$)
molecules in magnetically trappable states using a recently calculated
potential energy surface. We show that elastic collision rates are much faster
than inelastic rates for a wide range of fields at temperatures up to 10 mK and
that the ratio increases for lower temperatures and magnetic fields. If NH
molecules can be cooled to temperatures approaching 10 mK and brought into
contact with laser-cooled Mg then there is a good prospect that sympathetic
cooling can be achieved.
