[ 639 ] 



LX. The Intensity of the Earth's Penetrating Radiation at 

 Different Altitudes and a Secondary Radiation excited b\ y it. 

 By Prof. J. C. McLennan and E. N. Macallum, B.A* 



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 

 penetrating 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. 





Eatio. 



Ions per cm. 3 per see. 







1 



10 



1-00 

 •98 

 •83 



•80 

 •78 

 •67 

 •29 

 •008 



100 



1000 



•36 



•001 





The only numbers which directly lend confirmation to 

 Eve's conclusions appear to be given in a paper by Wulf (Le 

 Radium, June 1910, and Rhys. Zeit. 15 Sept., 1910), who 

 found "^" = 6 for the number of ions made per c.c. per 

 second by the penetrating radiation at the surface of the 



* Communicated by the Authors, having been read before the Royal 

 Society of Canada, May I (3th, 1911. 



