  We used FUSE to observe ultraviolet emission from diffuse O VI in the hot gas
in the Galactic halo. By comparing our result with another, nearby observation
blocked by an opaque cloud at a distance of 230 pc, we could subtract off the
contribution from the Local Bubble, leading to an apparent halo intensity of
I_{OVI} = 4680^{+570}_{-660} photons/cm^2/s/sr. A correction for foreground
extinction leads to an intrinsic intensity that could be as much as twice this
value. Assuming T ~ 3 x 10^5 K, we conclude that the electron density, n_e, is
0.01-0.02 /cm^3, the thermal pressure, p/k, is 7000-10,000 K/cm^3, and that the
hot gas is spread over a length of 50-70 pc, implying a small filling factor
for O VI-rich gas. ROSAT observations of emission at 1/4 keV in the same
direction indicate that the X-rays are weaker by a factor of 1.1 to 4.7,
depending on the foreground extinction. Simulated supernova remnants evolving
in low density gas have similar O VI to X-ray ratios when the remnant plasma is
approaching collisional ioinizational equilibrium and the physical structures
are approaching dynamical ``middle age''. Alternatively, the plasma can be
described by a temperature power-law. Assuming that the material is
approximately isobaric and the length scales according to T^(beta) d(ln T), we
find beta = 1.5+/-0.6 and an upper temperature cutoff of 10^{6.6(+0.3,-0.2)} K.
The radiative cooling rate for the hot gas, including that which is too hot to
hold O VI, is 6 x 10^{38} erg/s/kpc^2. This rate implies that ~70% of the
energy produced in the disk and halo by SN and pre-SN winds is radiated by the
hot gas in the halo.
