SECTION III., 1912. [3] Trans R.S. C. 
On the Relative Intensities of the Earth’s Penetrating Radiation on Land 
and Sea. 
By Proressor J. C. MCLENNAN. 
(Read May 16, 1912). 
I.—Introduction. 
Since the discovery of the existence of a penetrating radiation at 
the surface of the earth in 1902,* numerous measurements of its inten- 
sity have been made in different localities by different observers using 
a variety of measuring instruments. 
In these measurements the intensity of the radiation was generally 
found by determining the ionisation produced by it in the air confined 
in a metallic receiver. In some of these determinations care was taken 
to have the metallic receiver air tight, but in a number of them this 
precaution was not taken, either through its being considered unneces- 
sary or for other reasons. 
From the results of theseymeasurements it would appear:— 
(1). That the soil and rocks of the earth contribute by far the 
greater proportion of the radiation. 
(2). That the intensity of the radiation is less over such bodies of 
water as Lake Ontario, the Swiss Lakes, the Southern Atlantic and the 
Indian Ocean, than it is over land lying contiguous to these waters. 
(3). That the intensity of the radiation when measured at different 
levels above the ground in the neighbourhood of Braunschweig,! and 
on such structures as the Eiffel Tower,? and the City Hall Tower? at 
Toronto, decreases with the altitude of the point of observation, but 
when measured from the basket of a gas balloon at a height of over 
1000 metres above Vienna‘ is practically the same as it is at the sur- 
face of the earth in that locality. 

*McLennan and Burton, Phil. Mag. 5, 1903. 
Cooke, Phil. Mag. 6, 1903. 
1 Bergwitz Habilit-Schrift Braunschweig, 1910. 
? Wulf Phys. Zeit 11 p.811, 1910. 
3 McLennan & Macallum Phil. Mag., Oct. 1911. 
4 Hess Phys. Zeit 12 p. 998, 1911. 
