  Analytic and numerical approximations to a hydrodynamical model describing
longitudinally expanding, cylindrically symmetric, finite systems are fitted to
preliminary NA44 data measured in 200 AGeV central $S + Pb$ reactions. The
model describes the measured spectra and HBT radii of pions, kaons and protons,
simultaneously. The source is characterized by a central freeze-out temperature
of T_0 = 154 +/- 8 +/- 11 MeV, a "surface" temperature of T_r = 107 +/- 28 +/-
18 MeV and by a well-developed transverse flow, <u_t> = 0.53 +/- 0.17 +/- 0.11.
The transverse geometrical radius and the mean freeze-out time are found to be
R_G = 5.4 +/- 0.9 +/- 0.7 fm and tau_0 = 5.1 +/- 0.3 +/- 0.3 fm/c,
respectively. Fits to preliminary WA93 200 AGeV S + Au and WA98 158 AGeV Pb +
Pb data dominated by pions indicate similar model parameters. The absolute
normalization of the measured particle spectra together with the experimental
determination of both the statistical and the systematic errors were needed to
obtain successful fits.
