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In a series of high‐altitude cosmic ray intensity measurements, the technique of constant level flights with small extensible balloons was used. A reliable valve‐controlled system to level neoprene balloons with payloads of several kilograms at atmospheric depths between 4 and 10 g/cm2 was developed. The detector system consisted of a double coincidence GM counter telescope unit with telemetry of single counting rates, coincidence rate, atmospheric pressure, and internal temperature. Total payload weight was only 1500 grams. Thirty‐seven balloon flights were made during the last two years to study the cosmic ray intensity at high altitudes and low altitudes. A computing method was developed to fit a function of the type C (x, pλ) = A exp (−x/L) ‐ B exp (−x/l) to the counting rate versus atmospheric depth curve for each flight. This fit turned out to be excellent, leading to comprehensive information on the cosmic ray absorption, condensed into only three cutoff dependent parameters A, B, and L, and a constant parameter l. L represents the absorption length of the ionizing component; its mean value at 11.4‐gv geomagnetic cutoff rigidity was found to be 158 g/cm2. The parameter l, representing the apparent mean collision length, has a value of 86 g/cm2. This method provides a statistically accurate way to determine the extrapolated counting rate at the top of the atmosphere, given by the difference Co = A ‐ B. The correlation of the calculated counting rates at Pfotzer maximum and at the top of the atmosphere with the Mina Aguilar neutron monitor rate is analyzed for a period of 18 months of decreasing solar activity. A gradual softening of the primary spectrum during the eleven‐year cycle recovery is revealed.

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