  We use the continuity equation to derive a method for measuring the pattern
speed of the Milky Way's bar/bulge from proper motion data. The method has
minimal assumptions but requires complete coverage of the non-axisymmetric
component in two of the three Galactic coordinates. We apply our method to the
proper motion data from a combination of Gaia DR2 and VISTA Variables in the
Via Lactea (VVV) to measure the pattern speed of the bar as
$\Omega_\mathrm{p}=(41\pm 3)\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ (where the error
is statistical). This puts the corotation radius at
$(5.7\pm0.4)\,\mathrm{kpc}$, under the assumptions of the standard peculiar
motion of the Sun and the absence of non-axisymmetric streaming in the Solar
neighbourhood. The obtained result uses only data on the near-side of the bar
which produces consistent measurements of the distance and velocity of the
centre of the Galaxy. Addition of the data on the far-side of the bar pulls the
pattern speed down to $\Omega_\mathrm{p}=(31\pm
1)\,\mathrm{km\,s^{-1}\,kpc^{-1}}$ but requires a lower transverse velocity for
the Galactic centre than observed. This suggests systematics of
$5-10\,\mathrm{km\,s^{-1}kpc^{-1}}$ dominate the uncertainty. We demonstrate
using a dynamically-formed bar/bulge simulation that even with the limited
field of view of the VVV survey our method robustly recovers the pattern speed.
