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difficulties of our field conditions proved impossible. V\' e did collect slides within and outside of the plume from a hand held impactor slide of 0.6 cm width exposed for as long as a minute from a lightplane at 50 m/ sec. Microscope examination, under humidities varied by the operator's breath, showed that the hygroscopic particles had been collected, and the sizes were I'probably in the right ball park". The recommended calibration procedure is to examine the slides in the laboratory at very low relative humidity where the hygroscopic material will be visible as a solid. Another technique is to use the method employed by Farlow and French (1956). Tiny markers were put in the liquid from which droplets were generated. One type of marker used by R. E. Williamson and reported by MacCready and Todd (1964) consists of aspergillus niger spores, of size about 3 pm diameter. \Vith the initial liquid well stirred and the concentration of spores in it known, the mass of any impacted droplet could be found by counting markers, no matter what the distorted shape of the droplet was. The particles used for calibration should have a density comparable to that of the liquid in which they are distributed, and so the aspergillus niger spores may not be suitable for calibration of the hygroscopic material. The big worry in the hygroscopic spray system was that too much material would be wasted in large droplets, especially if the air pressure were somewhat low and the liquid flow rate somewhat high. By running the system on the ground with water, we got the qualitative feeling that. with the AiResearch compressor, a liquid flow rate of 7 gpm':< through the FTR nozzle system would give an adequate droplet distribution for seeding, although wasting some material. Lower liquid flow rates would decrease the percentage of wastage. We felt that in flight, at an average of 170 knots indicated air speed, large droplets would tend to break up. \Ve o angled the nozzles downward at about 45 to promote this breakup. ln flight with the hygroscopic material being dispensed the plume could be seen in the right illumination for about 20 seconds, and it remained as a rather narrow entity. Also, no faster falling material was evident. Thus the flight evidence, although weak, suggests that the amount being lost as large droplets was not great. It should also be noted that on several occasions in seeded areas the observation airplane encountered light rain which left a residue on the windshield, evidence of drops growing on rather large (say 100-200 pm dia.) hygroscopic particles. Later in the program we were able to use a stronger air source, the l\L~U\. putting out higher air pressure and volume flow, and the AF 100-nozzle dispenser. ':'gallons per minute 15