During the 1996 Hubble Space Telescope/Faint Object Spectrograph monitoring
campaign of the Seyfert 1 galaxy NGC 3516, the UV continuum showed a facto
r of 5 variation; correspondingly, the strongest broad UV emission lines (e
,g., Ly alpha, C Iv) displayed somewhat weaker, although still significant,
variations (a factor of 2). In contrast, no variation was detected in the
strong broad Mg II + UV Fe II emission line complex. While historically the
flux in the broad Mg rr emission line has varied (a factor of 2 within a f
ew years), the profile shape has not been observed to change over the past
decade. In contrast, the high ionization lines (HILs) show both emission-li
ne flux and profile changes on relatively short timescales, which appear to
be correlated with changes in the continuum level. Significantly, on 1996
February 21, the profile shapes of the HILs (e,g., Ly alpha, C Iv) and low
ionization lines (e.g., Mg II) were identical, at which time the continuum
level was at its highest. These results impose stringent constraints upon v
iable kinematic models of the broad-line region in NGC 3516, some of which
we discuss in this Letter.