  We use an N-body simulation to study the 3-D density distribution of spirals,
and the resulting stellar vertical velocities. Relative to the disc's rotation,
the phase of the spiral's peak density away from the mid-plane trails that at
the mid-plane. In addition, at fixed radius the density distribution is
azimuthally skewed, having a shallower slope on the trailing side inside
corotation and switching to shallower on the leading side beyond corotation.
The spirals induce non-zero average vertical velocities, <V_z>, as large as
<V_z> ~ 10-20 km/s, consistent with recent observations in the Milky Way. The
vertical motions are compressive (towards the mid-plane) as stars enter the
spiral, and expanding (away from the mid-plane) as they leave it. Since stars
enter the spiral on the leading side outside corotation and on the trailing
side within corotation, the relative phase of the expanding and compressive
motions switches sides at corotation. Moreover, because stars always enter the
spiral on the shallow density gradient side and exit on the steeper side, the
expanding motions are larger than the compressing motions.
