  Experiment NA57 has collected high statistics, high purity samples of
$K^0_s$, $\Lambda$, $\Xi$ and $\Omega$ produced in Pb-Pb collisions at 158 $A$
GeV/$c$. In this paper we present a study of the transverse mass spectra of
these particles for a sample of events corresponding to about the most central
55% of the inelastic Pb-Pb cross section. We analyse the transverse mass
distributions in the framework of the blast-wave model for the full sample
under consideration and, for the first time at the SPS, as a function of the
event centrality.
