308 



Mr. C. S. Wright on Variations in the 

 Table VIII. 



Distance of aluminium receiver from far wall =300 cms. 

 Distance of aluminium receiver from near wall = 105'5 cms. 

 Potential of receiver =250 volts positive. 

 Eadium placed on circle 100 cms. radius with receiver at centre. 



Distance of radium 

 from near wall. 



Ionization 

 (arbitrary scale). 



55 cms. 



51-86 



10-9 ' 



50-89 



20 



50-68 



41-4 



4995 



55-1 



49-13 



76-4 



4980 



110-9 



50-63 



153-7 



50-00 



197 



48-08 



experiments, it will be seen from Tables "VII. and VIII., 

 amounted to as much as 10 per cent. It seemed reasonable, 

 therefore, to suppose that the presence of some such object 

 as a brick wall might, when the penetrating rays from the 

 earth impinged upon it, in the same way affect the natural 

 ionization in any metallic cylinder; and care was therefore 

 taken in the measurements made when determining the 

 screening effect of the lake and of different soils, to place 

 the electroscope and receiver as far away as possible from 

 any building which might modify in some such way as that 

 indicated, the ionization of the enclosed air. 



5. Screening Experiments. 



Being assured from the foregoing experiments that changes 

 in the ionization, due to daily variations, were inconsiderable 

 in comparison with variations due to a change in position, 

 three series of measurements were then undertaken with the 

 object of investigating the screening effect of Lake Ontario, 

 care being taken to choose positions of observation as far as 

 possible from any artificial surroundings. The first and 

 second series were made with lead cylinders after the ioniza- 

 tion had reached the steady state, and the third with well- 

 cleaned cylinders of lead, zinc, and aluminium containing 

 freshly filtered air. A considerable decrease in the ionization 

 when measured over the water was noticeable in every case; 



