  We show how the continuity equation can be used to determine pattern speeds
in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine &
Weinberg in the context of external galaxies, requires projected positions,
$(l,b)$, and line-of-sight velocities for a spatially complete sample of
relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the
radial direction ($u_{\rm LSR}$), then the quantity that is measured is $\Delta
V \equiv \Omega_p R_0 - V_{\rm LSR}$, where $\Omega_p$ is the pattern speed of
the non-axisymmetric feature, $R_0$ is the distance of the Sun from the
Galactic centre and $V_{\rm LSR}$ is the tangential motion of the LSR,
including the circular velocity. We use simple models to assess the reliability
of the method for measuring a single, constant pattern speed of either a bar or
spiral in the inner MWG. We then apply the method to the OH/IR stars in the
ATCA/VLA OH 1612 MHz survey of Sevenster et al, finding $\Delta V = 252 \pm 41$
km/s, if $u_{\rm LSR} = 0$. Assuming further that $R_0 = 8$ kpc and $V_{\rm
LSR} = 220$ \kms, this gives $\Omega_p = 59\pm 5$ km/s/kpc with a possible
systematic error of perhaps 10 km/s/kpc. The non-axisymmetric feature for which
we measure this pattern speed must be in the disc of the MWG.
