  We use microwave temperature maps from two seasons of data from the Atacama
Cosmology Telescope (ACTPol) at 146 GHz, together with the Constant Mass CMASS
galaxy sample from the Baryon Oscillation Spectroscopic Survey to measure the
kinematic Sunyaev-Ze\v{l}dovich (kSZ) effect over the redshift range z = 0.4 -
0.7. We use galaxy positions and the continuity equation to obtain a
reconstruction of the line-of-sight velocity field. We stack the cosmic
microwave background temperature at the location of each halo, weighted by the
corresponding reconstructed velocity. The resulting best fit kSZ model is
preferred over the no-kSZ hypothesis at 3.3sigma and 2.9sigma for two
independent velocity reconstruction methods, using 25,537 galaxies over 660
square degrees. The effect of foregrounds that are uncorrelated with the galaxy
velocities is expected to be well below our signal, and residual thermal
Sunyaev-Ze\v{l}dovich contamination is controlled by masking the most massive
clusters. Finally, we discuss the systematics involved in converting our
measurement of the kSZ amplitude into the mean free electron fraction of the
halos in our sample.
