Work on this subject has recently been done by WALKER and PHILLIPS. In their experiments air was drawn by an electric fan through a channel bounded by a hot sheet of metal beneath, separated by about 6 mm. from a cold sheet of plate glass above. They produced polygons in air at rest, and longitudinal and transverse vortices when the fan was working; but as they pointed out there was a double shear in their apparatus which rendered their conditions not identical with those in the sky. Experiments with a single shear in liquids have been performed by IDRAC, MAL, and others ; longitudinal strips have been obtained and a transitional pattern, but no transverse rolls.