34 THE ROYAL SOCIETY OF CANADA 



The stratum immediately above the shale consisted partly of sand 

 and partly of clay. At one particular point this layer was about 2-5 

 metres thick, and it extended nearly up to the face of the opening 

 of the pit in the shale. At the very edge of the pit on the surface 

 of the shale the reading was 12 ions per cc. per sec, while on the 

 layer at a point about 6 metres back from the edge it was only 8 '67 

 ions per cc. per sec. This shewed that the layer of sand and clay 

 acted as a screen and cut off entirely the excess radiation from the 

 shale underneath. 



Another illustration of a screening effect was obtained in the 

 reading taken on the surface of the ground back about 30 metres 

 from the opening into the clay bank. At this station no direct radi- 

 ation could reach the instrument from the shale, and the radiation 

 from the clay deposits could only reach it after passing through the 

 surface layer of sandy loam, which was about one metre thick. The 

 reading at this station was only 6*82 ions per cc. per sec. From this 

 low value it was evident that the sandy loam contained considerably 

 less active matter than the clays and also that it acted as a screen 

 to absorb the radiation coming from the clay lying below it. 



It may be added in regard to these various deposits that it was 

 the shale which was used in making the red-coloured bricks and the 

 clays that were used in making the light-coloured ones. 



V. On the penetrating radiation from the atmosphere. 



In several previous communications by one of us and by C. S. 

 Wright attention has been drawn to the fact that the ionization in 

 a closed vessel is very much less over the water of Lake Ontario than 

 it is at points on the neighbouring land. Further, in measurements 

 made on the lake with a Zinc receiver of about 27 litres capacity and a 

 Wilson electrometer the value of "q" has been shewn to be about 4-46 

 ions per cc. per sec. 



On repeating these measurements with a Wulf Electrometer the 

 mean of a number of sets of observations gave a value of 4 «93 ions 

 per cc. per sec, for "q" which is slightly higher than the value previously 

 obtained, with the larger receiver. These observations were carried 

 out on a small skiff about 2 kilometres from the shore at a point where 

 the water was about 20 metres deep. 



In a second set of observations made at this point on the skiff 

 the electrometer was placed for protection in a metal box of alu- 

 minium bronze with walls a little over one centimeter thick. In 

 these measure ments, the value of 4.81 ions per cc per sec was ob- 

 'q' 



