[MCLENNAN-MACALLUM] EARTH’S PENETRATING RADIATION 45 
5. Assuming that Eve’s value of 1-6 ions per c.c. per second 
represents the direct effect of the penetrating radiation it follows that 
2-6 ions per c.c. per second must be produced by the secondary rays 
emitted by the walls of the zine receiver or by these and by other radia- 
tions which may be emitted by the soil and which have not yet been 
detected. 
The writers wish to acknowledge their indebtedness to Mr. $. A. 
Kennedy for kindly assisting in making a number of the observations 
described in the paper. 
The Physical Laboratory, University of Toronto, 
May 10th, 1911. 
