  We have carried out a detailed analysis of the IUE archival high resolution
spectra of the classical nova V1974 Cyg 1992. In addition to the P Cygni and
emission lines, two shortward shifted absorption systems are present. Evidence
is given that these absorptions originate in two separate expanding shells,
outside the wind layers where the emission lines are formed. The outer main
shell, containing most of the matter ejected at outburst, produces the
so-called ``principal absorption line system'', and the inner faster moving
second shell produces the so-called ``diffuse--enhanced absorption line
system''. The outflow velocity of the two shells increases exponentially with
time reaching a value of about 1750 km/s and 2900 km/s, respectively. We
suggest that the acceleration of the shells is the result of increasing
line-radiation pressure due to the UV-brightening of the star as the effective
radius decreases. Around day 60 the second shell has overtaken the slower
moving principal system shell, and merged with it. This explains: the sudden
disappearance of the diffuse line system near that date, the upward jump of 240
km/s in velocity of the principal system and the first detection of hard X-ray
emission on day 63. This velocity jump indicates that the main shell is about 4
times more massive than the second shell. The deceleration suffered by the
diffuse-enhanced system after the shock provides a shock temperature of about
1.6 KeV, in fairly good agreement with the temperature of the observed hard
X-ray emission.
