LONG-RANGE FALLOUT FROM SEDAN AND SMALL BOY 85 length of this sampling line was 170 miles and extended northward from Antelope to Mayoworth, Wyo. Since the trajectory of the cloud from the explosion could not be predicted accurately enough to permit preshot installation of sampling sites, this line had to have the capability of being rapidly deployed after the shot when better meteorologi- cal data were available. Rapid deployment was accomplished by the sampling crew with a DC-3 aircraft and local commercial automobiles. Therefore this line of stations was called “the mobile line.” The mobile line for the Small Boy shot extended from Walsenburg in southern Colorado to Brighton in the northern part of the state. A second line of 11 stations, about 1000 miles from ground zero and approximately 1100 miles long, extended from Abilene, Tex., to Williston, N. Dak. A third line of nine stations, 1400 miles from ground zero and 1100 miles long, extended from Jackson, Miss., to Inter- national Falls, Minn. First-order U. S. Weather Bureau stations along these lines were used as sampling sites for both the Sedan and the Small Boy shots. Since these sites were not movable, they were re- ferred to, respectively, as the first and second line of fixed stations. INSTRUMENTATION Each site on the mobile and the first fixed line was equipped with a Staplex high-volume air sampler and a fallout collector. The stations on the second fixed line were provided with only the air sampler. The air sampler is shown in Fig. 2 with an 8- by 10-in. filter holder. A TFA No. 41 ashless filter paper was used which is rated at a 95% collection efficiency for particles as small as 1 uw in diameter. The units were adjusted to a flow rate of about 50 cu ft/min at the start of the sampling period. The actual flow rate at subsequent times was read from a small rotameter affixed to the rear of the sampler. After the initial adjustment the flow rate was recorded every 6 hr for a total of five readings over a 24-hr period. The paper was replaced, and the cycle was repeated until three 24-hr samples were collected. The total volume sampled by each filter paper was determined by plotting these readings as a function of time and integrating the area under the curve. The fallout collector consisted of a galvanized tray 42 in. long, 33 in. wide, and 3 in. deep (see Fig. 3). The surface of the collector was about 18 in. aboveground and sloped gently toward the center of the tray where a ¥,-in. hole drained into a 5-gal polyethylene-lined drum beneath the collector. The capacity of the drum was equivalent to 0.83 in. of rain falling on the collecting surface. The active area of the fallout collector was coated with a thin layer of white petroleum jelly to present a tacky surface. Theoretically, the tacky surface would