[mclennan-mcleod] EARTH'S PENETRATING RADIATION 35 



In a third set the metal box containing the electrometer was lowered 

 into the water to a depth of about eight metres, and in these experiments 

 the average reading obtained was 4-77 ions per cc. per second. From 

 this result it would appear that the radiation coming from the air above 

 the lake did not contribute any more to the ionisation in the electro- 

 meter than about '16 ions per cc. per sec. 



In attempting to ascertain the contribution made to the ionisation 

 in a closed vessel by radiation coming from active matter in the air 

 above the land, measurements are complicated by the effect of radiation 

 from active matter in the soil and from active matter deposited on the 

 surface of the ground from the atmosphere. It would seem, however, 

 since the measurements described above were made at a point only 

 about 2 kilometres from the shoie, that the contribution made to the 

 ionisation in air in a zinc vessel by radiation from the active master 

 in the air over the land near Toronto cannot be very far from 0-2 ions 

 per cc. per sec. 



VI. Residual Ionisation. 



From the measurements which have been described above it 

 would appear that the lowest value obtainable for the ionisation in a 

 closed zinc receiver at points near the surface of the earth where the 

 intensity of the penetrating radiation may be considered to have a 

 minimum value, approximates to 4 ions per cc. per sec. This it will 

 be remembered was the mean value obtained for q from the measure- 

 ment made on the SS. Hesperian on Sept. 21st, 1912, and it was also 

 practically the value found for "q" by Simpson and Wright* on a skiff 

 on the Southern Atlantic. 



To what then is this residual ionization due? As an attempt to 

 answer this question the following experiments may be of interest. 

 In these the electrometer was placed in the aluminium box used in the 

 experiment under water on the lake and this box was suspended in a 

 moderately-sized tank which could be filled with water and which was 

 situated in the attic of the Physical Laboratory at Toronto. Readings 

 were taken of the ionisation in the electrometer when the tank was empty 

 and also when it was filled with watei . With the tank empty the read- 

 ing was 8'05 ions per cc. per sec, and with it filled the reading was 

 5-82 ions per cc. per sec. The dimensions of the box were 36 x 22 x 18 

 cm. and those of the mass of water surrounding it were 121 x 90-7 x 83 

 cm. This it will be seen gave roughly a mean of 36 '5 cm. as the thick- 

 ness of the water screen surrounding the box. 



*Loc. cit. 



