diameter and 5/15 in. high), positioned in a quadralateral about 2i in. ona side. The cups were coated on the inside with silicone grease to produce a tacky surface, This tacky surface held almost all particles which came in contact with it; rainwater collected during the sampling interval would not wash particles from the tacky surface un~ less the particles themselves were soluble. An 8-oz jar was fastened beneath an opening in the bottom of the tray to collect liquid fallout (Fig. 2.4). A doar covered the sampling opening both before and after the sampling time (Fige2.5), The instrument was started by an external timing signal. After a delay of 1 min,the cover door opened and the first tray mved into sampling position. Succeeding trays moved into position under the cover opening at set time intervals until the cycle was completed; (Fige 206). The door then closed and the nachine shut itself off. At the time of the detonation an external timing signal actuated self-latching signal relay R, (Fig. 27). Current then flowed through the clock which had been pre-set for a short time delay before the door opened (Fig. 2.8). At the end of this delay microswitch in the clock was tripped, allowing the current to flow through the driving motor which in turn rotated the spider; the door opened and tray 1 moved into sampling position. Sirce microswitch S3, underneath the spider rim was nc longer closed by one of the cams on the spider, microswitch C5 opened. ‘This removed the current fron the clock coil and reset the clock. The driving motor continued to run until the cam under the next tray moved over 53, When S3 closed, the current path to the drivfng motor was broken and the motor stayed off until the clock finished another cycle. Succeeding trays moved into vosition under the cover cpening at set time intervals until the sampling cycle was completede . At the time of detonation a spring cam was resting on a microswitch S,, completing the circuit through the contact points of elec- trical latching relay Ra. As the cycle progressed, the spring cam rode over the microswitch, S5, completing a circuit through R3, which was thrown and latched. After the last tray was in sampling position and the door closed, the spring cam again rode over S,, breaking the circuits and stopping the instrument. Push-button switch, S,, was used as a reset syvritch so that the operator could easily reset the entire instrument by one simple operation. Toggle switch S5 was mounted umer the clock and was used to preset the glock. This switch remained closed during the entire operation. Resistor Ny controlled the driving motor speed to keep the trays from overshooting their positione Variable l-ohm resistors and also lengths of nichrome wire were used. 2.3 TIMING Where wire timing signals were available at a station, a minus l-sec signal supplied by Edgerton, Germeshausen, and Grier (EG&G) was used to actuate the IFC. Where no wire timing signals were available at a station, an EG%G Mark TII or Mark IV battery-powered bluebox was used to actuate the IFC. Wire timing signals were initially used, 18