Many of the data were read from plots, others were available in tabular form. The particles were placed in size bins, and the specific activities were averaged. Hence in the plots that follow, each point may represent from 2 to 20 particles. Data from King are plotted in Figure 4. Only particles smaller than 5 um appear to have been measured. The large rate of increase of specific activity with decreasing particle size is evident. Data from Lassen are shown in Figure 5. They do not lend themselves readily to interpretation. This may be the result of the fact that this shot had an anomalously low yield. Data from Priscilia found so far were only for "fallout" particles between 500 and 3,000 um and were gamma activities at 14 days. Since particles that large are not representative of balloon shots or airbursts, we suspect that radioactivities induced in the soil were measured. Data from Wilson are based on unpublished work by Nathans done during the early seventies. Cloud samples were subjected to a size separation by combined gravitational settling and sequential centrifugation, and the specific abundance of a number of nuclides was determined in the size fractions. In addition, the fraction of fused particles was determined by microscopy. By dividing the values of this fraction into the radionuclide specific abundances in the size fractions, the average specific abundances in the fused particles which contain most of the radioactivity are obtained. The results for plutonium are shown in Figure 6. The size range extended from 0.08 um to 10 um. No increase below a few micrometers is evident. The data below 1 um are somewhat tenucus, however, because of the large uncertainties associated with them. Tower Shots Gross activity data have been analyzed from three Greenhouse shots, four Tumbler-Snapper shots, one Upshot-Knothole shot, and two Plumbbob shots. The scaled heights of burst varied from 30 feet to 200 feet, the yields from 5 kt to 40 kt. The particles covered the range from about 1 um to about 1 cm, although the size ranges for individual shots were much more limited. Plutonium data were analyzed for one shot from the UpshotKnothole series. The gross activity data are plotted in Figures 7 through 9. Figure 7 is a composite plot showing the general behavior of the specific activity with size. Figures 8 and 9 show the specific activity of Tumbler-Snapper particles below 50 um (T-S6) or below 25 um (T-S5, 7, 8). These Tumbler- Snapper data have been plotted separately so as not to crowd any more points into Figure 7. The straight lines drawn in Figure 8 are leastsquares regression lines, as are the lines drawn through the various sets of points below 20 um in Figure 7. The slopes of these lines vary 572