fined to one station only, showed that the deposition rate increased after the thermonuclear tests in the Pacific in 1954 and maintained a mean value of 2.3 mc/km*/year throughout the period ending April 1956. This result was shown to be consistent with the fact, established independently, that the debris from large-scale nuclear explosions is stored in the stratosphere and is returned to earth slowly over a number of years. With only one sampling station it was not possible to deal with the important questions of the uniformity of this deposition over the U. K. and over the earth’s surface. A network of six stations in the U. K. and thirteen in other parts of the world has now been set up, at which rain water is collected over three-month periods and analyzed for Sr®*, sr®, ce'*, and Cs". Some of these stations have been in operation since the middle of 1955 and an accountis given in this report of the results obtained so far. The non-uniformity observed in the world pattern of deposition shows that such global surveys are essential for a true assessmentof the fallout problem, while the results as a whole throw someinteresting light on the mechanism whereby fission products are transferred from the stratosphere to the troposphere and deposited on the ground by rain water. Table 1— LIST OF RAIN WATER COLLECTION STATIONS Station Mean annual Longitude Kinloss 57° 39'N OF 34°W 70 January 1956 Liverpool Snowdon* Abingdon 53° 21/N 53° 04'°N 51° 41’N 51° 41’N 02° 58’°W 04° O1°W o1° 18°W 05° 09°W 85 300 65 January 1956 October 1956 July 1955 Felixstowe 51° 58’N OL 20'E 55 January 1957 Bodo Esquimalt 67° 17°N 49° 30'N 14° 22°E 123° 00°W 87 79 July 1957 October 1957 July 1955 Milford Havent Tromso 69° 42'N rainfall, ems Sampling Latitude 19° OVE 75° 41°W 75 commenced May 1954 68 June 1957 Ottawa 45° 20’N 100 Gibraltar 36° 10’N 65° 21°W 90 Caenwood 18° 13'N 76° 35°W 280 July 1957 Palisadoes 17° 56’°N 76° 47°W 80 July 1957 Port Harcourt Singapore 04° 45'N 01° 19°N 07° 20’°E 103° 49°E 250 240 Suva 18° 05’8 178° 28°E 290 Melbourne Ohakea 37° 45°S 40° 12’S 144° 50’E 175° 23/E 65 100 Port Stanley 51° 42’8 57° 52°W 65 July 1956 January 1956 July 1955 July 1955 —: January 1956 July 1955 February 1956 * Two stations one 335 m higher than the other. {1 Monthly and three-month samples. 2 NETWORK OF RAIN WATER COLLECTION STATIONS A list of the stations in current operation, arranged in order of latitude is given in Table 1 together with the dates when sampling commenced. The period of sampling at all stations is three months (January to March, April to June, etc.) but the monthly sampling at Milford Haven, which was started in May 1954 has been continued to provide somefiner detail. The stations in the mountainous region near Snowdon were chosen because the rainfall there is about four times the average for the U. K.; one of the two Stations is at an altitude 335 metres above the other. The recently commissioned stations in the West Indies, Caenwood and Palisadoes, were selected because they have quite different rainfall patterns although they are close together geographically. Sampling at latitudes north and south of 60°N and 55°S, respectively, is difficult because of the lack of suitable stations and because of the problems associated with snow sampling. Through the kindness of T. Hvinden of the Norwegian Defence Research Establishment, snow cores and samples of summer rain have been obtained from latitudes up to 70°N. Through the 232 ipearsageatas