IN his Friday evening discourse at the Royal Institution on February 8, Sir Gilbert Walker discussed natural and artificial clouds. Apart from cumulus clouds of various types, the causes of the geometrical patterns that are to be seen in the sky must be sought in the behaviour of layers of fluid which are made unstable either by heating them from below or cooling them from above. It has been known for fifteen years that a stationary liquid when unstable develops a polygonal pattern, and that an unstable liquid flowing down a trough forms pairs of vortices rotating in opposite directions, with their axes parallel to the direction of flow, or of shear. Sir Gilbert's pupils have carried these investigations further, and A. Graham used a wind tunnel formed with a heated iron plate as lower surface; its upper surface was a cool glass strip 8 ft. long and 9 in. wide, a third of an inch above the iron plate. When pulled by a motor, this gave variable rates of shear in the air. The former explanation of clouds occurring in long rolls or in a rectangular pattern as caused by Helmholtz waves was shown to be unsatisfactory; and it was verified that while a rapid shear due to motion exceeding one inch a second produces longitudinal cells, one less than a fifth of that rate leads to transverse cells, and an intermediate rate to a rectangular pattern. Various types of longitudinal clouds were discussed, and Sir Gilbert withdrew his former explanation of spirals in these clouds as due to stream lines, showing that there are normally two equidistant spirals like those in a twistdrill, and that these appear to be the two component vortices intertwined. Photographs of clouds were used to demonstrate the formation of a number of patterns of clouds at different heights; and an account was given of the explanation suggested by A. Graham of the paradox that, in the laboratory, liquid rises in the axis of a cell while in air there is descent there. Attention was also directed to the existence of clouds due to instability or the sun; and to the use of clouds of longitudinal and rectangular patterns for longdistance gliding under the name of ‘cloud-streets’.