The instability of a rectangular solid in plane strain subjected axially to a constant pressure p and laterally to a constant hydrostatic pressure q is investigated; a modification of the problem is treated in reference [1] in which the lateral load q was assumed to be constant directional. It was found that, unlike the constant directional q, the hydrostatic pressure q stabilizes the solid, but with increasing q/p, the destabilizing effect due to the caused deformations predominates. It is also shown that a number of investigators, who concluded that beams will not buckle when q = p, based their conclusions on formulations which are not valid in this range and that their conclusions are not correct.