  In this paper we present the analysis of two broad band (0.1-150 keV)
BeppoSAX observations of the Seyfert 1 galaxy NGC 3516. The two observations
were taken 4 months apart, on 1996 November 8 and 1997 March 12 respectively.
We report a dramatic change in the degree of obscuration of the central source
between the two observations, and propose, as possible explanations transient
absorption by either a stationary-state cloud of cold gas crossing the line of
sight, or a varying-state, initially neutral and dense amount of expanding gas
with decreasing density and therefore decreasing opacity. We also report the
detection of a second highly ionized absorber/emitter, which causes deep
FeXVII-XXII K edges at about 7.8 keV to appear in both BeppoSAX spectra of NGC
3516, and possibly produces the soft X-ray continuum emission and the 1 keV
blend of Fe L recombination lines detected during the epoch of heavy nuclear
obscuration.
