196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
body had thus far assumed. What was needed was absorption 
experiments to determine just what sort of rays they actually were. 
We carried 300 pounds of lead and a 6 by 6 by 6 foot tank of water 
to the top of the peak and obtained as the net result of these absorp- 
tion experiments the definite proof that the rays found at the top of 
Pike’s Peak were predominantly of the hardness of ordinary gamma 
rays, and further that they were very largely, if not entirely, of local 
origin, since local conditions, such as a heavy snow storm and bliz- 
zard, which occurred while we were there, varied their intensity 
nearly as much inside a screen of 4.8 centimeters of lead as outside. 
Kolhorster had by this time, after the brief publication of our Kelly 
Field data, and as a result, also, of new experiments made subse- 
quently to them in crevasses and holes in glaciers in the Alps, reduced 
his estimated absorption coefficients ®° from 0.57 to 0.25, a change he 
regards as within the limits of his experimental uncertainties, but a 
change which made the assumed rays so hard as to be no longer 
irreconcilable with our sounding balloon observations. But we found 
that our Pike’s Peak observations were not yet compatible with his 
now (1923) assumed characteristics of rays of cosmic origin, viz., 
rays which produce 2 ions per second per cubic centimeter at the 
earth’s surface, and have a coeflicient of 0.25 per meter of water. For 
while in going from the altitude of Pasadena to that of Pike’s Peak 
the number of ions observed with the unshielded electroscope in- 
creased from 11.6 to 22.2, an increase of 10.6 ions, the number of ions 
observed through the Shield of 4.8 centimeters of lead increased but 
from 9.37 to 11.6, an increase of only 2.23 ions. But radiation of the 
characteristics assumed above would have caused by itself, inside our 
lead screen, an increase of 3.34 ions, even if none of the large increase 
in radiation shown by the unshielded observations got through the 
lead shield—a supposition which we believed to be contrary to fact. 
In a word, our Pike’s Peak observations showed that if rays of cosmic 
origin existed at all they must be of different characteristics from 
any as yet suggested, and they further showed most interestingly 
that a very copious soft radiation of unknown origin existed at the 
altitude of Pike’s Peak. 
Accordingly, Mr. Harvey Cameron and myself planned some new 
experiments for the summer of 1925 which were designed: 
(1) To settle definitely the question of the existence or non- 
existence of a small, very penetrating radiation of cosmic origin—a 
radiation so hard as to be uninfluenced by, and hence unobservable 
with the aid of, such screens as we had taken to Pike’s Peak—and, 
(2) To throw light on the cause of the variation with altitude of 
the radiation of gamma-ray hardness which our absorption experi- 
® Kolhorster, Sitz.-Ber. Preuss. Akad. Wiss., 34, 366, 23. 
