  The EIT waves [named after the Extreme-ultraviolet Imaging Telescope (EIT)
onboard the Solar and Heliospheric Observatory (SOHO)] are in the literature
usually described as fast magneto-acoustic (FMA) modes. However, observations
show that a large percentage of these events propagate with very slow speeds
that may be as low as 20 km/s. This is far below the FMA wave speed which
cannot be below the sound speed, the latter being typically larger than $10^2$
km/s in the corona. In the present study it is shown that, to account for such
low propagation speed, a different wave model should be used, based on the
theory of gravity waves, both internal (IG) and surface (SG) ones. The gravity
modes are physically completely different from the FMA mode, as they are
essentially dispersive and in addition the IG wave is a transverse mode. Both
the IG and the SG mode separately can provide proper propagation velocities in
the whole low speed range.
