  We have detected faint, diffuse H$\alpha emission from several points along
the Magellanic Stream, using the Rutgers Fabry--Perot Interferometer at the
CTIO 1.5-m telescope. At points on the leading edges of the H I clouds MS II,
MS III, and MS IV, we detect H$\alpha emission of surface brightness $0.37 \pm
0.02$ Rayleighs, $0.21 \pm 0.04$ R, and $0.20 \pm 0.02$ R respectively,
corresponding to emission measures of 1.0 to 0.5 \cmsixpc. We have observed
several positions near the MS IV concentration, and find that the strongest
emission is on the sharp leading-edge density gradient. There is less emission
at points away from the gradient, and halfway between MS III and MS IV the
H$\alpha surface brightness is $< 0.04$ R.
  We attribute the H$\alpha emission at cloud leading edges to heating of the
Stream clouds by ram pressure from ionized gas in the halo of the Galaxy. These
observations suggest that ram pressure from halo gas plays a large role in
stripping the Stream out of the Magellanic Clouds. They also suggest the
presence of a relatively large density of gas, $n_{\rm H} \sim 10^{-4}
cm^{-3}$, in the Galactic halo at $\sim 50$ kpc radius, and far above the
Galactic plane, $b \sim -80\deg$. This implies that the Galaxy has a very large
baryonic, gaseous extent, and supports models of Lyman-$\alpha and metal-line
QSO absorption lines in which the absorption systems reside in extended
galactic halos.
