A NEUTRON source (beryllium and 300 millicurie radon) was placed in the centre of a block of paraffin wax, of about 20 cm. diameter in all dimensions. After emerging from the paraffin, the neutrons had to pass, on their way to tho detecting apparatus, through the marginal parts of a wooden disc of 50 cm. diameter and 2 cm. thickness, which could be made to rotate with a speed of 4,000 rev. per minute. This speed corresponds to a velocity of about 9 × 103 cm./sec. of the parts of the wood through which the neutrons penetrated. From former experience one would expect each slow neutron, while passing through the disc, to suffer a number of collisions with the protons contained in the wood, and therefore the velocity of the disc would be superimposed on their velocity distribution with the disc at rest. Assuming that a large part of the neutrons have thermal velocities, which are for neutrons of the order of 2 × 105 cm./sec, an asymmetrical distribution of the directions of the neutrons emerging from the rotating disc would result, about ten per cent more slow neutrons being emitted in the direction of rotation than in the opposite direction.1