  We have used moderate-resolution (FWHM ~ 25 km/sec) spectra of AGNs and QSOs
observed by the Far Ultraviolet Spectroscopic Explorer (FUSE) to make the first
definitive measurements of absorption by hot gas in high velocity clouds (HVCs)
at large distances from the Galactic plane. Seven of the 11 sight lines studied
exhibit high velocity (|V(LSR)| > 100 km/sec) O VI 1031.93 A absorption with
log N(O VI) = 13.79-14.62. High velocity O VI absorption is detected in the
distant gas of H I HVC Complex C, the Magellanic Stream, several HVCs believed
to be in the Local Group, and the outer Galaxy. The fraction of O VI in HVCs
along the seven sight lines containing high velocity O VI averages about 30
percent, with a full range of 10-70 percent. The O VI detections imply that hot
(log T(K) = 5-6) collisionally-ionized gas is an important constituent of the
HVCs since O VI is difficult to produce by photoionization unless the path
lengths over which the absorption occurs are very large (>100 kpc). The
association of O VI with H I HVCs in many cases suggests that the O VI may be
produced at interfaces or mixing layers between the H I clouds and hot, low
density gas in the Galactic corona or Local Group. Alternatively, the O VI may
originate within cooling regions of hot gas clouds as they are accreted onto
the Galaxy.
