A shallow layer of silicone oil on a plane, circular copper plate was uniformly heated from below. The air on its surface was kept at constant temperature by a uniformly cooled glass plate close to the oil, which at the same time inhibited air motions. Motions began with concentric circular rolls which, after the centre ring had formed, broke down into a hexagonal pattern. A rather accurate determination of the wavelength of the motions was possible. The wavelength was found to be variable with the depth of the fluid layer in qualitative accordance with the theory of Nield. Supercritical motions are briefly discussed.