  We show the initial results of our 3D MHD simulations of the flow of the
Galactic atmosphere as it responds to a spiral perturbation in the potential.
In our standard case, as the gas approaches the arm, there is a downward
converging flow that terminates in a complex of shocks just ahead of the
midplane density peak. The density maximum slants forward at high z, preceeded
by a similarly leaning shock. The latter diverts the flow upward and over the
arm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again,
generating further secondary shocks as it approaches the lower z material.
Structures similar to the high z part of the gaseous arms are found in the
interarm region of our two-armed case, while broken arms and low column density
bridges are present in the four-armed case.
  We present three examples of what can be learned from these models.
