Under the general heading of Reactor Lattices are reported a new method of calculating resonance escape probabilities, results for the effective mass of a proton bound in the water molecule, and a comparison of similar techniques for calculating lattice constants developed independently by Hanford physicists and by a school of Russian physicists (as reported at the Geneva Conference). An estimate of maximum errors in f and η for thermal systems is described. Experiments and analysis thereof on the effects of neutron streaming in air channels through a moderator are reported. Buckling calculations and experimental results are given for graphite lattices employing 1.17-inch diameter natural uranium slugs and U²³⁵-Aluminum alloy slugs. An experimental measurement of the critical mass of an annular cylindrical array is described. Some measurements on, and the status of, construction of the Lattice Testing Reactor are reported. Under Instrumentation, development of a BF/sub 3/ counter suitable for operation at elevated temperatures is described. Some measurements on, and the status of, construction of the Lattice Testing Reactor are reported. Under Instrumentation, development of a BF₃ counter suitable for operation at elevated temperatures is described. A method of determining the screening parameter in the Thomas-Fermi model of the atom is …