analysis provides the cumulative deposition up to the time the soil has been collected. Rain water sampling provides a current record of how much Sr®* has been deposited in a given time interval. The rain water (funnel, pot, or tub) technique assumes that practically all the Sr”? comes downin precipitation, a fact that seems to be fairly well established or, perhaps more accurately, that a pot really collects about the same amount as is deposited on the soil, Finally, the use of air filtration techniques also provides a record of current rather than cumulative amounts of Sr®’, It is also obvious that the air concentration is not necessarily a good measure of the amount of Sr®*’ that is being deposited out. There are suggestions that the removal of Sr*? particles by natural precipitation is more effective at the altitudes at which the natural clouds occur than at the ground level. 3 SOIL Figure 1 shows the latest available results of soil sampling in various parts of the globe in 1956. The points have been taken in both Americas, Europe, Asia, Australia, and Africa, mainly 30 TOT x PPT 25 ka 20 2 = 9 Hewtens: Sy 15 = eo” a 40 5 oO 30° 70" 60° 50° 40° 30° 20° 40° Q° NORTH LATITUDE 40° 20° 30° 40” 50° 60° 70°90° SOUTH LATITUDE Fig. 1——Soil data, 1956: ©, electroanalysis x 1.15; x, HCl extraction, by Dr. Alexander of the Department of Agriculture. The electrodialysis technique apparently does not extract all the radiostrontium out of the soil. Comparison with HC] extraction shows a large variability in ratio of the results of the two techniques, but an average increase of about 00 per cent is suggested for the electrodialysis as noted on the figure. It is apparent that a large scatter of points exists at any one latitude. The main reason for this scatter is probably the real differences in fallout due to precipitation differences and other meteorological factors. Secondary reasons are the errors in the extraction of the strontium from the soil and counting of the radioactivity. (Other causes such as run-off of water in heavy rain, penetration of depths below the 2 in. sampled, etc., are undoubtedly present but are probably small compared to the first two classes of errors.) The heavy line is an attempt to construct a single north-south average profile. Dr. Libby has provided estimates of the amount of tropospheric fallout of Sr® from all tests. Table 1 shows this estimate of the number of megatons equivalent Sr*® fallout from the U.S.S.R., U. S. (Nevada), U. S. (Pacific), U. K. (Pacific), and U. K. (Australia) tests. The lower part of Fig. 2 shows a north-south profile for the Upshot-Knothole test series in the spring of 328