-14Tungsten-185, with a half-life of 74 days, was produced in a number of detonations over silica sand. By studying this isotope it is hoped that the origin of the middle northern hemisphere maximum of strontium- 90 fallout can be clarified, Three main models of world-wide transport have been proposed, each giving a different interpretation of this observed phenomenon. Reduced to their simplest terms, they are: (1) A model, described by Libby (Ref. 8), based on uniform distribution of radioactive debris in the stratosphere. In this model the non-uniformity of fallout is due to non-uniform rainfall and to the contribution from lowyield tests which deposit debris in the troposphere. The low-yiéld tests have been mostly conducted in the middle and high latitudes of the northern hemisphere. This model, then, attributes a major part of the strontium-90 fallout which had occurred in the United States by late 1957 to Nevada tests and USSR low-yield tests. (2) The second model, described by Machta (Ref. 6), is based on a very slow mixing rate, both horizontally and vertically in the lower stratosphere, together with a slow poleward circulation in both hemispheres. In this model material from both equatorial Pacific tests and high-latitude USSR tests tends to drift towards higher latitudes and to enter the troposphere non-uniformly with a maximm in the middle latitudes of both hemi spheres in late winter and spring. If this model is correct, the major part of the strontium-90 fallout in the United States up to early 1958 had come from tests in the equatorial Pacific. (3) DOE ARCHIVES A third model, recently proposed by Martell (Ref. 16) is based on a shorter stratospheric residence time for debris injected at high latitudes than at equatorial latitudes. This model attributes a large part of the (le