A fluid is contained between rigid horizontal planes which move relative to each other with constant horizontal velocity. A gravitationally unstable temperature difference is maintained at the boundaries, and the heat flux and momentum flux (stress) transmitted by the fluid are measured. The Nusselt number, Nu, and the dimensionless momentum flux, Mo, are obtained for small mean rates of shear. The Rayleigh number, R, and the Prandtl number, σ, are both large in these experiments. The data are consistent with the following relations:\[Nu \propto Mo \sigma^{\frac{1}{2}} \propto R^{\frac{1}{3}}.\]

Kraichnan's mixing-length theory of turbulent thermal convection is extended to the present situation, and the above experimental dependence of Nu and Mo on R and σ is obtained. The agreement between mixing-length theory and experiment provides strong support for Kraichnan's concise treatment of turbulent convection. The importance of this result is that heat and momentum fluxes may be calculated in this way for a variety of flows in geophysics.