  We measured Reynolds numbers R_e of turbulent Rayleigh-Benard convection over
the Rayleigh-number range 2 times 10^8 < R < 10^11 and Prandtl-number range 3.3
< sigma < 29 for cylindrical samples of aspect ratio Gamma = 1. For R < R_c = 3
times 10^9 we found R_e ~ R^beta_eff with beta_eff = 0.46 < 1/2. Here both the
sigma- and R-dependences are quantitatively consistent with the Grossmann-Lohse
(GL) prediction. For R > R_c we found R_e = 0.106 sigma^-3/4 R^1/2, which
differs from the GL prediction. The relatively sharp transition at R_c to the
large-R regime suggests a qualitative and sudden change that renders the GL
prediction inapplicable.
