The bottom of Willis’ box did not admit of down motion by underflow, nor did the piston pressure create an opposed horizontal force that might have come from the ocean area. In restraining up motion over the folds that formed, Willis piled bags of shot on top of the model to represent downweighting. The folding in the Appalachians was down at the bottom of the heap where things were hot and compressed, and heat could extend individual strata. In our pressure chest we extended the Willis conception. We made two pistons at opposite ends of an oaken box, with thick plate glass panes at one side, so as to watch the folding. The two pistons would distribute the end pressure better and admit the possibility that all the pressure did not come from the continent. The bottom under the model was an inner box that could move down, hung on heavy spring balances. These could be screwed up to a pressure upward to compensate the load of shot. Thus the first fold could arch downward as well as upward. This imitated a possible lowered trough bottom. The piston rate of advance was controlled by metronome, one man at each screw. For examples, models E, F, and G had four white and four black layers, all alike in substance, at fast, medium, and slow rates. The quickest was shortened one inch in five minutes. The slowest was one inch in an hour and three-quarters. The quick-squeeze model flexed smoothly, all folds seemed to flow, and the model held together compactly. The slow-squeeze model shortened the same amount, cracked in many places, was brittle, and did not hold together compactly. This appeared to prove that slow motion will fracture where quicker motion will hold strata intact, under otherwise identical conditions of substance, of folding and shortening, and of vertical confinement. We verified Willis’ conclusions that stiff and thick beds transmit the pressure farthest and that overthrust tends to form in soft beds, which thicken near a piston. In one model we got overthrusts in opposite directions on opposite sides of the model along a single-fold axis, with a twist in between. While an experiment was in progress, the chest creaked occasionally, the equivalent of an earthquake. One model was cast to represent overlap of strata near shore, like a coastal plain. When squeezed, it made a group of overthrusts away from the piston acting as shore rock.