courtesy of Dr. Lister of the British Transantarctic Expedition, snow cores have been obtained from 78°S 37°W where summer melting is negligible, and arrangements have been madeto obtain fresh snow samples from 82°N to 70°W, 3 SAMPLING PROCEDURE At each station rain water is collected in a 4.5 liter polythene bottle through a polythene funnel with a 5 cm rim and a diameter of 10 cm or 20 cm according to the mean rainfall. When the collecting bottle is nearly full, or at the end of a sampling period, the water is transferred to a polythene transit bottle and returned to the laboratory, where carriers are added and radiochemical analysis carried out. An exception is made in the case of the monthly collector at Milford Haven where carriers are added before the rain falls, and the good agreement between the results obtained for Sr® on the monthly and three-monthly collector on the same site has served as a check onthe reliability of the latter system for sampling this isotope. The radiochemical procedures have been described in another report.” On certain sites particularly, it has been found that the quantity of rain water collected does not correspond with that collected on the immediately adjacent standard rain gauge. The tendency has been for the polythene funnel to collect rather more than the rain gauge and discrepancies as high as 25 per cent have been found. It is probable that this phenomenon will occur with most types of collector. The procedure adopted here is to compute the specific activity of the rain water using the volume of water in the collector and then to compute the fallout per unit area using the rainfall figures from the standard rain gauge. 4 SOME GENERAL FEATURES OF THE DEPOSITION PROCESS It is necessary at this stage to discuss some general features of the deposition process which are relevant both to the sampling method used andto the treatment and interpretation of the data obtained. The first concerns the relative importance of the two ways in which very fine dust is removed from the atmosphere: (1) by washout in rain water; (2) by dry turbulent deposition onto surfaces. A previous report! quoted evidence to show that the former was by far the more important process, particularly in the case of dust fed down from the stratosphere. Nevertheless, if a collecting funnel is exposed to the atmosphere for a lengthy period, significant amounts of radioactive dust will be deposited on the inner surface by turbulent deposition, and if the amount of rain during the period of exposure is small, the specific activity of the water in the bottle will be must higher than that of the falling rain. Some experiments have been carried out which allow an empirical correction to be made for this effect when the concentration of activity in ground level air is known throughout the period of exposure. In the first place, the deposition of fission product activity onto horizontal sheets of gummed paper was measured daily over a period of weeks together with the concentration of activity in the air at ground level. The deposition velocity Vg defined by Vg(cms/sec) = rate of deposition of activity per cm” per sec concentration of activity per em! of air was found to have a mean value of 0.07 cm/sec which compared well with the mean value obtained by Megaw for the deposition of small particles onto filter papers.° It is reasonable to expect that the nature of the surface is unimportant in the case of very fine particles, provided there are no large electrostatic charges. In a separate series of experiments, one of the standard polythene collecting funnels was lined on the inside with gummed paper and the deposition of fission products compared with that on flat sheets of gummed paper erected alongside. Over a period of twenty days it was found that the amount deposited on the inside of the funnel was equivalent, within 3%, to that on a flat sheet of paper of area equal to that of the cross-section of the funnel. Thus the dry rate of deposition of airborne activity onto the inside of the polythene funnel may be also taken to be 0.07 cm/sec. 233 aay