measured by a recorder (Esterline-Angus Model AW), plotting radiation intensities from 0.005 to 100 mr/br from the lowlevel photomultiplier tube, and from 10 mr/hr to 200 r/hr from the high-level photomultiplier tube. . Since-only three decades of information are to be presented at a time, additional load resistance, R-31, B-32, are added in series with the internal resistance of the recorder. The R-31 is set at 1,850 ohm from arm to ground so that a change of one volt with respect to ground is equal to a radiation change of 3 decades. The three decadé scales of the Lk decade response, of each photomultiplier tube is selected by subtracting a fixed voltage in series with theoutput signal. The four voltages are provided by potentiometers R-35, 36, 37 and 38. They are consecutively selected by switch 8-1 in positions A,B,C, and D. The switch also connects the proper grid bias, buck-out voltage and high voltage into the circuit for the selected scale. On any scale, the voltage at the output of the subtraction circuit will be zero volts for the minimum intensity and one volt for the maximum intensity. In the OFF position of S-1, the batteries are disconnected. Those remaining in the circuit are utilized at negligible current drain. In the SET position, the recorder is used as a voltmeter to check the B plus voltage. 2.1.5 Altitude Compensation Since the distance from the aircraft to the ground is continuously varying, the aircraft readings are useful only when referred to a standard distance from the surface. For the evaluation