  We have determined the projected rotational velocities of thirteen blue
horizontal-branch (BHB) stars in the globular cluster M13 via rotational
broadening of metal absorption lines. Our sample spans the photometric gap
observed in the horizontal branch distribution at Teff ~= 11000 K, and reveals
a pronounced difference in stellar rotation on either side of this
feature---bluewards of the gap, all the stars show modest rotations, vsini < 10
km/s, while to the red side of the gap, we confirm the more rapidly rotating
population (vsini ~= 40 km/s) previously observed by Peterson et al (1995).
Taken together with these prior results, our measurements indicate that a
star's rotation is indeed related to its location along the HB, although the
mechanism behind this correlation remains unknown. We explore possible
connections between stellar rotation and mass loss mechanisms which influence
the photometric morphology of globular cluster HBs.
