vertical. In most of the surveys at the Eniwetok Test Site the westward drift of the radioactivity was approximately equal to that of the current speed, although in some instances it was lower. This may be attributed to the effect of daily vertical migration of the macroplankton into deeper water, where the current flow is slower than that at the surface, and to eddyeffect, if this exists. A measure of the transport by surface currents of radioactivity in the Pacific Ocean was reported by Miyaxn, Suciura and Kamepa (6), by Harvey (7), and by SEYMouR ef al. (4). Miyake et al. reported that 4 months after the nuclear tests at Eniwetok and Bikini the radioactivity had moved west to a distance of 1,200 miles, which +t is equal to a drift of more than 9 miles per day. Harley reported the westward drift to extend approximately 4,300 miles west from Bikini, which gives approximately the same rate of drift. In an oceanic survey made with the USS Marsh approximately six weeks after the 1956 test series at the Eniwetok Test Site (Seymouret al.), the highest observed centre of radioactivity was derived from a surface detonation at Bikini Atoll. Forty-three days before the Marsh survey, the centre of this activity was located almost due north of Bikini Atoll. During the intervening time the contaminated centre moved a total of 300 miles at an average rate of approximately 7 miles per day. The radioactive material in this area of high activity was almost entirely associated with the surface waters (above 25 m), and the direction of drift (WSW)of the contaminated water from the original site, at which it wasfirst detected, was in the direction of the prevailing winds in that area. Thus the major part of the radioactivity was carried by the surface currents in the direction of the wind and remained in the surface water for at least a month anda half. The original contaminated deeper waters from this detonation appear to have moved almost due west, and according to the arsh survey contained low levels of activity in comparison with the surface waters. However, the deeper waters may haveinitially contained appreciable amounts of radioactivity which by the time of the second survey had sunk to a depth beyond the depth of sampling (300 m). Revelle and Schaeffer reported that the rate of vertical diffusion above the thermocline is about 1,000 times that of molecular diffusion and about 1,000 times less than that of horizontal dispersion. The extent of vertical stirring within the mixed layer depends upon the surface windsandthevertical density gradient. The thermocline is a layer of rapid temperature change and separates the surface layer, which in the area of the Eniwetok Test Site is Jess than 100 m thick, from the deeper waters. Because the temperature is fairly consistent throughout the upper or mixed layer, mixing in this layer should occureasily and require only small amounts of energy. Because the thermocline is a layer of high stability, transfer of materials across this layer by turbulent diffusion would be expected to be much less rapid than in the upper layer. Therefore radioactive materials introduced above the thermocline should remain in the mixed layer for a long time and be subjected to great horizontal distribution and small amounts of vertical distribution. However, radioactive materials may cross the thermocline by at least two methods: 1. If the radioactive material is accompanied by large amounts of stable elements, such as calcium compounds, as where weaponsare fired over coralline islands or reefs, the calcium may precipitate into particles of 108 Fee cage ert neonannpRT TD Tay pay 1 VET Wh