mine the rate at which activity built up on the surface at these locations, we used the relationship between activity and granule size previously described. Equation (2) is a simple model to describe fall time versus gran- ule size. The bulk of the activity of BRAVO was at the base of the cloud at 17 to 29 km (10 to 20 miles) aboveground, ten minutes after the burst (0C68). Granules of a given size were spread throughout the stem, the base of the cloud, and up to the cloud top at 40 km (25 miles). In fact, the entire distribution of granule sizes would reach the surface at any point in time, not just one size at one time. Our simple model (Eq. (2)), which we assumed for our purposes of estimating the rate-of-rise of exposure rate, the rate of accumlation of activ~ity at the surface, and the accumulated external exposure during the period of rising exposure rates, was in agreement with measurements on rate-of-rise of ex- posure rate for weapons tests made during the Hardtack Series in 1957 (USPHS59) and with the rate-of-rise of exposure rate measured for one half-hour at Rongerik Atoll on March 1, 1954 (S055). These estimates of granule fall time, granule size, and activity versus granule size were combined in a straightforward manner to determine the cumulative percent of activity deposited on the surface of the Sth Lucky Dragon versus time after the BRAVO explosion. The cumulative percent is plotted in Figure 5. The fallout was first observed by the fishermen on the Lucky Dragon at 3 hours post-detonation. Examination of Figure 5 indicates that the bulk of activity had fallen on the fishing vessel by % hours post-detonation. Granules could no longer be seen falling by the crew of the 5th Lucky Dragon at 8 hours post-detonation (T355). The Rongelap people who were interviewed at the time of evacuation indicated to Sharp that the granules were noticed first at 5 hours post- detonation (Sh57). Bikini Atoll, These people were about 210 km (130 miles) from Nam Island, the origin or center of BRAVO fallout. Duration of the fallout was observed to be about 7 hours at Rongelap Island (Sh57). Using Eq. (2), we estimated granule size for Rongelap Island on the basis of observed fallout ar- rival and cessation times. The Rongelap granule size distribution was assumed to have the same shape as Bikini ash. Assuming that the activity of a granule was proportional to the 3.5 power of the granule size, we estimated the percent of total activity versus granule size'at Rongelap (see Figure 6). For fallout at Rongelap Island, the size corresponding to median activity was about 150 \m (6 x 1079 in.). The cumlative percent of total activity deposited on the sur- face of Rongelap Island versus time post-detonation (see Figure 7) was estimated by us using the same method described here for Bikini ash. Fallout was not visible at Utirik Island. The first analysis of arrival time of BRAVO fallout, based on an assumed mean wind speed, was estimated by Sondhaus et al. to be 22 hours post-detonation (Cr56). Fallout cesSation was estimated to be 34 hours post-detonation. We estimated new values for fallout arrival and cessation times at Utirik Island on the basis of observations made by persons on the 5th Lucky Dragon, on Rongelap Island, and the military outpost on Rongerik Atoll. Fallout was first seen at 150 km (90 miles) at 3 hours post-detonation by the Japanese = 25 -