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By the use of bomb-produced tungsten-185 tracer, debris from the 1958 nuclear tests (Hardtack) held at the U.S. Pacific Proving Ground have been identified as they appeared in the ground-level air along the 80th meridian. A large amount of radioactivity from these tests appeared in South America, particularly at the high-altitude collecting stations.

Early in 1948 the observation was made by personnel of the U. S. Naval Research Laboratory that gamma counters located on the ground at various places in the Northern Hemisphere showed a marked increase in gamma count whenever rain fell. This indicated the possible use of rainfall as a scavenger for airborne radioactive matter. It was known that there had been some atomic bomb tests in the Pacific and we suspected that the increase in count could be due to fission products, although the possibility existed that it could be due to the natural radioactivity always present in the atmosphere. In order to determine specifically what the activity was due to, it was necessary to obtain rain water that had been collected during the early months of 1948. A source of such water was discovered in the Virgin Islands where the entire water supply comes from rain. A large volume (2500 gallons) of this rain water from a cistern was treated with aluminum-hydroxide floe. Positive identification of the fission products Ce141, Ce144, and Y91 in the floe proved that the increase observed was due at least in part to fission products. A similar experiment using cistern water and plant material from Truk Atoll gave even higher fission product counts. Following this, experimental sites were set up in Washington, D. C., and Alaska early in 1949, and routine rain water col-

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