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The galactic fountain theory predicts that hot gas from superbubbles escapes from the Galactic disk, cools radiatively as it rises upward, and eventually recombines and returns to the disk ballistically as high-velocity H~{\sc i} clouds (HVCs). I am completing surveys of HVCs in nearby disk galaxies and of X-ray emitting superbubbles in the disk of M33.
Fourteen nearly face-on disk galaxies were observed with the Arecibo 305~m telescope; the double-horned H~{\sc i} profiles have high-velocity wings in ten of these galaxies. Such wings can be caused by HVCs similar to those observed in our own Galaxy. The galaxies with no detected high-velocity wings are also those with the lowest far-infrared fluxes as measured by IRAS , which is consistent with supernovae providing the kinetic energy of the HVCs. I observed NGC~5668 and UGC~12732 with the Very Large Array, and these observations confirm the H~{\sc i} profiles observed with Arecibo. In NGC~5668 there are two main components of high-velocity material, one of which is associated with a larger H~{\sc i} velocity dispersion within the optical radius of the galaxy, while the other may be the remnant of a past interaction. UGC~12732, which has little high-velocity gas, has an H~{\sc i} velocity dispersion half that of NGC~5668.
M33 has 148 holes in the neutral hydrogen disk, many of which are associated with H {\sc ii} regions and OB associations. Such holes may be caused by dense supershells surrounding expanding superbubbles of low-density hot gas. The handful of superbubbles that have been observed in the Galaxy and in the Large Magellanic Cloud have X-ray luminosities of $10^{35}-10^{38}$ ergs~s$^{-1}$ and diameters of 100$-$1400 pc. A 35 ksec ROSAT High-Resolution Imager (HRI) observation found 27 X-ray sources more luminous than 6~$\times$~10$^{36}$ erg~s$^{-1}$ within 17\farcm5 of the nucleus of M33, one of which is close to the edge of the field of view and is associated with an H~{\sc i} hole and the giant H~{\sc ii} region IC 133. The X-ray luminosity of this potential superbubble is 10$^{38}$ ergs~s$^{-1}$ and the diameter is about 300~pc. Further HRI observations to survey the entire disk of M33 for superbubbles were performed in 1994~Jul$-$Aug and will be presented.
