ACKNOWLEDGEMENTS The authors wish to acknowledge the assistance of the many people who have worked on this program since its inception. It is impossible to name all the individuals involved but our particular thanks are given to renry the observers of the various U. S. Weather Bureay Stations, the Canadian Meteorological Service, the Air Weather Service of the Air Force, and to the others who collected the samples over this period of years, In the Health and Safety Laboratory we are grateful to the gummed film staff for preparing and counting the enormous number of samples involved and to the staff of the Stetistics branch for handling the card punching, computation and printing of data, The strontium 90 analyses which have been used for comparison were made under the general direction of Edward P., Hardy, Jr. and Gerald Hamada. Considerable general assistance has been available from Dr. Lester Machta and Robert List of the U, S, Weather Bureau who arranged for many of the sampling locations and maintained liaison between the Weather Bureau and our Laboratory. Dr. S, Allan Lough, Director of the Health and Safety Laboratory, po made many valuable suggestions in the preparation of the manuscript. Fr A h. = « 4 +r - ili - k r F r I h I t b br be be k a ¥ p rs e % Mn. t * a