[McLENNAN-MACALLUM] EARTH’S PENETRATING RADIATION 41 
observations the readings in the laboratory were 1-7 ions per cc. 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 approximately 
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 connection with 
their observations is that the readings which they obtained 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 Pacinif in connection with his obser- 
vations 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 upon 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 5-4 
ions, per c.c. per second, and so make the readings at Toronto, Livorno 
and Matjesfontain practically the same. 
It would seem then that 5-8 ions per c.c. per second, may be taken 
as representing approximately the ionization produced under normal 
conditions by the earth’s radiation in air confined at atmospheric 
pressure in a zinc receiver. 
Since Eve has shewn that 1-6 ions per c.c. per second is all that 
can be estimated as being produced directly by the penetrating rays 
* Proc. Roy. Soc. No. A: 577, p. 175, 1911. 
+ Annali dell’ Ufficio Centrale Meteor. e. Geod. Italiano, Vol. XX XII, 1910, pt. 1. 
