  This paper describes some interesting properties of waves in, and
oscillations of, the interstellar medium in the direction normal to the plane
of the Galaxy. Our purpose is to examine possible reasons for four observed
phenomena: the falling sky in the northern hemisphere; the apparent presence of
clouds in absorption spectra when a sightline is occupied primarily only by
warm intercloud gas; the peculiar structuring of spiral arms involving clumps,
spurs, and feathering; and the existence of an abundance of high stage ions far
off the plane of the Galaxy.
  We explored the reaction of the interstellar medium - in the vertical
direction only - to large imposed disturbances (initial displacements,
expansive velocities, and compressions), and to the introduction of small
amplitude waves via oscillation of the midplane. Our findings included: 1) the
anticipated growth in amplitude of high frequency waves with height; 2) the
four lowest normal modes for the oscillation of the atmosphere as a whole, as
functions of the height of the outer boundary; 3) the time for material to
`bounce' from one unusually dense state to the next as a function of height;
and 4) the tendency for the disk to develop a hot outer halo, either after the
passage of a single shock from a large event, or in response to a continuous
stream of small amplitude waves.
