The heat transferred by free convection through vertical plane layers of water and mercury was measured as a function of the temperature difference across the layer, the height of the layer, and its thickness. Subsequently, the reduction in the heat transferred through the mercury due to the application of transverse magnetic fields was measured and correlated by the parameter (magnetic field intensity squared)/(square root of the layer temperature difference). The form of this correlation was determined by an analysis based upon the assumption that the flow within the layer resembled two boundary layers, one rising on the hot side and the other falling on the cold side, with crossflow at the top and bottom of the layer.