From abstract: "Critical assembly tests have been made on the Pajarito Universal Machine to determine the safety limits of handstacking cubes of 95% U-235 in spherical geometries preparatory to making critical assemblies of U-235 in an "infinite" tuballoy tamper. The U-235 cubes were stacked as pseudospheres in a cavity at the top of a tuballoy block 12" high and 8" on a side, and the assembly tamped by locating it inside a doughnut shaped water tank whose tamping effect is equivalent to the tamping caused by personnel necessary for hand stacking."