We propose to search for far-infrared emissions from shock-heated
 water in extremely high states of rotational excitation.  The water
 transitions considered in this proposal possess upper state
 energies (E/k) between 2300 and 4700 Kelvin and thereby provide a
 unique probe of the very hottest molecular regions that are present
 within the interstellar medium.  The study of such highly-excited
 transitions is complementary to - but DOES NOT FORM PART OF  -
 THE GUARANTEED TIME PROGRAMME.  Because of the low luminosities
 that are expected for the water lines to be studied in this project,
 we will search for them in Cepheus A (West), a source of luminous
 shock-excited H2 line emission which shows a relatively small infrared
 continuum flux in the spectral region where the target water
 transitions lie.  Our choice of transitions is motivated by a
 theoretical model for the excitation of water emission in molecular
 shock waves, constructed by Kaufman and Neufeld.   That model,
 in common with previous studies, predicts that water will be
 copiously produced in the warm region behind the shock front
 and that FAR-INFRARED WATER ROTATIONAL EMISSIONS WILL DOMINATE
 THE COOLING OF DENSE, NON-DISSOCIATIVE MOLECULAR SHOCKS.  The
 high-lying transitions that we propose to study will provide a
 more sensitive diagnostic of the physical conditions in the
 water-emitting region region than will the lower-lying rotational
 lines to be studied in the Guaranteed Time Programme.  The high
 atmospheric opacity at the far-infrared wavelengths of these
 transitions, and the expected weakness of the emission lines,
 necessitate the use of ISO for the project proposed here.