Earth's Penetrating Radiation at Different Altitudes. G43 



measurements the ground was bare. This difference in the 

 condition of the ground might easily account for the difference 

 in the results. Further support for this explanation is found 

 in the fact that in the present investigation the readings ob- 

 tained in the physical laboratory were practically the same 

 as those obtained on the bare lawn, while in Wright's obser- 

 vations the readings in the laboratory were 1*7 ions per c.c. per 

 second higher than those obtained on the lawn, when covered 

 with ice and snow. 



The conclusion that 5'8 ions per c.c. per second is approxi- 

 mately a measure of the normal intensity of the earth's 

 penetrating radiation is also supported by some readings 

 recently made (near Cape Town) by Simpson and Wright *, 

 who are at present attached to the Scott Antarctic Expedition. 

 In the course of some observations made by them with an 

 apparatus similar to that used by us, they found a difference 

 of 6* ions per c.c. per second between the readings taken 

 inland some 200 miles at Matjesfontain, and those taken on 

 the ' Terra Nova ' on the open sea. 



A further point of interest which they noted in connexion 

 with their observations was that the readings which they ob- 

 tained on the 'Terra Nova' due to the penetrating radiation 

 immediately on leaving land, gave a mean about 3 ions per 

 c.c. per second higher than those they obtained some time later 

 when the vessel was well out to sea. This increase they 

 ascribed to the presence of radioactive matter which came 

 from the atmosphere over the land and was deposited on the 

 surface of the vessel when lying in port. 



This effect may possibly account for a difference between 

 some numbers recently published by Pacini f in connexion 

 with his observations near the naval station at Livorno in 

 Italy, and those obtained at Toronto and Cape Town. In his 

 measurements he found a difference of only 2'4 ions per c.c. 

 per second between the readings taken on the land near the 

 shore and those taken on a launch on the sea some 300 metres 

 from the shore. 



The effect due to active deposits on the launch would 

 naturally depend to some extent on the size of the latter, but 

 on the basis of Wright's observations, one may perhaps 

 legitimately attribute 3 ions per c.c. per second to such active 

 deposits. This would make the ionization due to the radiation 

 from the land at Livorno equal to 5'I ions per c.c. per second, 



* Proc. Roy. Sec. A. No. 577, p. 175 (1911). 



t Annali delV Ujficio Centrale Meteor, e. dead. Italiano. vol. xxxii. 



1910. pt. 1. 



