  We used the time correlation of shadowgraph images to determine the angle
$\Theta$ of the horizontal component of the plume velocity above (below) the
center of the bottom (top) plate of a cylindrical Rayleigh-B\'enard cell of
aspect ratio $\Gamma \equiv D/L = 1$ ($D$ is the diameter and $L \simeq 87$ mm
the height) in the Rayleigh-number range $7\times 10^7 \leq R \leq 3\times
10^{9}$ for a Prandtl number $\sigma = 6$. We expect that $\Theta$ gives the
direction of the large-scale circulation. It oscillates time-periodically. Near
the top and bottom plates $\Theta(t)$ has the same frequency but is
anti-correlated.
