  We present Far Ultraviolet Spectroscopic Explorer observations of the atomic
and molecular absorption in high velocity cloud HVC 287.5+22.5+240, which lies
in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We
detect H2, N I, N II, Si II, and Fe II absorption and set limits on the amount
of absorption due to P III, Ar I, and Fe III. We extend the earlier metallicity
and dust-depletion measurements made by Lu and collaborators by examining the
relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived
gas-phase abundances due to ionized gas in the HVC are less than 15%. The HVC
has a metallicity of 0.2-0.4 solar, similar to that of the Small Magellanic
Cloud. The relative abundance pattern for the elements studied resembles that
of warm gas in the SMC, which supports the idea that this HVC is part of the
tidally stripped Leading Arm of the Magellanic Stream. The abundance pattern
implies that the HVC contains dust grains that have been processed
significantly. It is likely that the grain mantles have been modified or
stripped back to expose the grain cores. We have identified more than 30 lines
of H2 arising in the HVC from rotational levels J = 0 to J = 3. Synthetic
spectra and a curve-of-growth fit to these lines with b = 12 km/sec indicate
that log N(H2) = 16.80+/-0.10 and f(H2) = 2N(H2)/[N(H I)+2N(H2)] = 0.0016. From
an analysis of the H2 rotational populations, we find an absorption rate (at
1000 A) that is less than one-tenth the average value in the solar
neighborhood. The presence of molecular gas in the HVC requires that either the
H2 formed in situ or that molecules formed within the SMC survived tidal
stripping. We favor the latter possibility because of the long formation time
derived for molecules in this HVC.
