  We present an investigation of the ultraviolet and X-ray spectra of the
Seyfert 1.5 galaxy Markarian 817. The ultraviolet analysis includes two recent
observations taken with the Cosmic Origins Spectrograph in August and December
2009, as well as archival spectra from the International Ultraviolet Explorer
and the Hubble Space Telescope. Twelve Ly-alpha absorption features are
detected in the 1997 GHRS and 2009 COS spectra - of these, four are associated
with high-velocity clouds in the interstellar medium, four are at
low-significance, and the remaining four are intrinsic features, which vary
between the GHRS and COS observations. The strongest intrinsic absorber in the
1997 spectrum has a systemic velocity of ~ -4250 km/s. The corresponding
feature in the COS data is five times weaker than the GHRS absorber. The three
additional weak (equivalent width from 13-54 mA) intrinsic Ly-alpha absorbers
are at systemic velocities of -4100 km/s, -3550 km/s, and -2600 km/s. However,
intrinsic absorption troughs from highly ionized C IV and N V, are not detected
in the COS observations. No ionized absorption signatures are detected in the ~
14 ks XMM-Newton EPIC spectra. The factor of five change in the intrinsic
Ly$\alpha$ absorber is most likely due to bulk motions in the absorber, since
there is no drastic change in the UV luminosity of the source from the GHRS to
the COS observations. In a study of variability of Mrk 817, we find that the
X-ray luminosity varies by a factor of ~40 over 20 years, while the UV
continuum/emission lines vary by at most a factor of ~2.3 over 30 years. The
variability of the X-ray luminosity is strongly correlated with the X-ray
power-law index, but no correlation is found with the simultaneous optical/UV
photometry.
