  We report new observations of the far infrared and submillimeter water vapor
emission of NML Cygnus based on data gathered with the Infrared Space
Observatory and the Submillimeter Wave Astronomy Satellite. We compare the
emission from NML Cyg to that previously published for VY CMa and W Hya in an
attempt to establish the validity of recently proposed models for the outflow
from evolved stars. The data obtained support the contention by Ivezi\'{c} &
Elitzur (1995, 1997) that the atmospheres of evolved stars obey a set of
scaling laws in which the optical depth of the outflow is the single most
significant scaling parameter, affecting both the radiative transfer and the
dynamics of the outflow. Specifically, we provide observations comparing the
water vapor emission from NML Cyg, VY CMa and W Hya, and find, to the extent
permitted by the quality of our data, that the results are in reasonable
agreement with a model developed by Zubko & Elitzur (2000). Using this model we
derive a mass loss based on the dust opacities, spectral line fluxes, and
outflow velocities of water vapor observed in the atmospheres of these
oxygen-rich giants. For VY CMa and NML Cyg we also obtain an estimate of the
stellar mass.
