HIGH FREQUENCY RAYS—MILLIKAN 195 
in ordinary electroscopes at the surface. The war put a stop the 
world over to further studies of this sort, but as soon as we could 
get the proper instruments built after the war in the newly equipped 
Norman Bridge Laboratory of Physics, I. S. Bowen and myself went 
to Kelly Field, near San Antonio, Tex., as with four little record- 
ing electroscopes which we succeeded in the spring of 1922 in sending 
up in sounding balloons to almost twice the heights which had previ- 
ously been attained. The highest flight reached the altitude of 
15.5 kilometers, or nearly 10 miles. 
These instruments were interesting in that, though they were built 
of steel to hold 300 cubic centimeters of air at 150 pounds pressure, 
and were provided each with a recording barometer, thermometer, 
and electroscope, also with two different sets of moving photographic 
films and the necessary driving mechanism, the total weight of the 
whole instrument was yet but 190 grams, or about 7 ounces. The 
altitudes were determined not only from the now well-established 
law of ascent of balloons, but also by direct, two theodolite observa- 
tions which Maj. William R. Blair of the United States Signal Corps 
kindly sent Lieutenant McNeil to Kelly Field for the express pur- 
pose of making for us. 
In these experiments we expected, if the results previously re- 
ported were correct, to find very large rates of discharge; for our 
instruments went up to such heights that 88 per cent of the atmos- 
phere had been left beneath them, and only 12 per cent was left to 
cut down, by its absorption, the intensity of the hypothetical rays 
entering from outside. In other words, our electroscopes should have 
been exposed to radiations approaching in intensity those existing 
at the very top of our atmosphere. We actually failed to find any- 
thing like the computed rates of discharge. Our experiments were 
in agreement with those of the European observers in that our 
electroscopes showed a somewhat higher rate of discharge at high 
altitudes than at the surface, but at the same time they proved 
conclusively that a radiation of the assumed properties did not exist, 
our observed rates of discharge being not more than one-fourth the 
computed amounts. 
Since the origin of the “ penetrating rays” was still uncertain, Dr. 
Russell Otis and myself in the summer of 1923 went to the top of 
Pike’s Peak for the sake of making absorption experiments upon 
this radiation at the highest altitude to which we could carry large 
quantities of absorbing materials. For if the rays were not of cosmic 
origin they did not need to be more penetrating than are the gamma 
rays from radioactive materials, while if they were of cosmic origin 
the sounding balloon experiments of Bowen and myself had shown 
that they must be very much harder (more penetrating) than any- 
