In the atomic gas, as well, thermal and mechanical conditions are intimately related. Consider, for example, the gas distribution above and below the Galactic disk midplane. Under the influence of the vertical gravitational force, the two components should separate spatially, with the colder and denser medium residing closer to the plane. Indeed, we may estimate the scale height of the cold neutral medium by balancing gravity against the internal pressure gradient. Let be the Galactic gravitational potential, so that V$s is the vector force per unit mass. The equation of hydrostatic equilibrium then reads
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