432 GUDIKSEN ET AL. pole contact switches mounted on the fixed bulkhead is activated by a cam on the rotating inlet plate, thus opening the circuit to the drive motor, which stops the rotation. The main power switch is then turned off. Airtightness is maintained at the cannister inlet by applying spring pressure on specially designed Teflon seals placed between the rotating plate and the fixed bulkhead. Silicone grease applied to the seals helps to prevent air leakage as well as to lubricate the moving parts. Leak tests were performed on unexposed cannisters filled with molecular-sieve material. After a 4-hr flight, the CO, recovered from the sieve amounted to 0.05 liter-atmosphere of CO,. Since this amount is comparable to the amount of CO, absorbed by the molecular sieve during its handling in the laboratory, leakage during flight is insignificant. Flow-rate calibrations have begun but have not yetbeen completed. For a 45-min sampling period, we have obtained about 8 liter-atmo- spheres of CO, at an altitude of 29,000 ft and approximately 5 liter- atmospheres at 39,000 ft. The results seem tobe reproducible to within +10% from flight to flight. In addition, v= have tested the CO, absorption efficiency by sectioning the cannisters into two parts and analyzing each part separately for absorbed CO, content. We found that only about 3% of the total amount of CO, was absorbed in the back quarter section, thus demonstrating that virtually all the CO, passing through the cannister is absorbed by the molecular sieve. RESULTS The gross fission-product gamma-ray spectrum ofa typical stratospheric sample collected in April 1964 is shown in Fig. 4. The data were obtained with a 3- by 3-in. Nal crystal and a 512-channel analyzer. The individual activities of Mn, ®Zr— Nb, '"cs, !SRu—!Rh, 1255p, and ‘Ce were determined by spectrum stripping with the use of a computer. The *Zr—®*5Nb activity is plotted vs. the collection date in Fig. 5. The top curve represents the activity of stratospheric samples, and the lower curve represents the activity of tropospheric samples. The activities are expressed in terms of picocuries per 45-min sampling time. Conversion to conventional units based on 1000 scf of air will be made when the calibration of air flow rates are completed. Preliminary results indicate that the volume of air at standard temperature and pressure sampled in the stratosphere is about five-eighths of the volume of air sampled in the troposphere. The numbersindicate the sampling altitude in thousands of feet. The curve at the bottom of Fig. 5 represents the height of the tropopause on the sampling date.