FIG. 15 Three possible gas reservoirs: On the left (a), a crevice (extended perpen-dicular to the plane of the page) or conical pit. On the right (b), a small-mouthed re-entrant cavity. In each case, the opening radius is a. FIG. 16 A thin sheet of liquid advancing from right to left encounters a crevice. If θ<2β, then the crevice