SECTION ITI., 1911. [37] Trans. R.S. C. 
The Intensity of the Earth’s Penetrating Radiation at Different Altitudes 
and a Secondary Radiation Excited by it. 
By Pror. J. C. McLennan and Mr. E. N. Macauuvm, B.A. 
(Read May 16, 1911.) 
I.—PENETRATING RADIATION AT DIFFERENT ALTITUDES. 
In a paper in the Phil. Mag. Jan. 1911, page 37, Dr. A. S. Eve on 
certain assumptions calculates the probable effect of altitude on the 
intensity of the 7 rays from radium C present in the earth and obtains 
the values given in Table I. below for the ionization due to this radiation 
at points at different heights above the ground. 
In his calculations he made no correction for a decrease in the 
density of the air with increase in altitude, but pointed out that this 
would tend to augment the lower figures given in the table. He also 
stated in the paper that the number of ions made per cubic centimetre 
per second should perhaps be doubled if the effect of thorium in the 
earth were added to that of radium. 
From the numbers given in the table it is clear that if the penetrat- 
ing radiation present at the surface of the earth is entirely of terrestrial 
origin, it should be possible to detect a diminution in the intensity of 
this radiation even at moderate distances above the earth’s surface. 
TABLE I. 
Height in Metres. Penetrating Radiation. 
Ratio. Ions per em.$ per sec. 
0 1-00 80 
12 ES lds RSR SU re -98 78 
(eer SOUS TO OR CEE pena seer ols -83 67 
Use ee dore RE RER -36 29 
OR RE ben e ali qu -001 008 
The only numbers which directly lend confirmation to Eve’s con- 
clusions appear to be given in a paper by Wulf (Le Radium June 1910, 
and Phys. Zeit. 15 Sept. 1910), who found “q” = 6 for the number of 
ions made per cc. per second by the penetrating radiation at the surface 
