[MCLENNAN-MACALLUM] EARTH’S PENETRATING RADIATION 39 
TABLE II. 
‘‘q’? == the number of ions made in the receiver per cubic centimetre per second. 
The ice on 
The lawn, Toronto Bay. Basement of City Hall 
The Physical University. Depth of water City Hall. Clock Tower. 
Laboratory. Campus. 5 metres. Engineer’s 64 metres 
Distance from room. high. 
shore — 400 
| metres. 
15-2 15-1 9-3 16-5 | 11-9 
15-3 15-2 9-2 15-9 12-2 
15-5 15-0 8-9 15-2 11-6 
14-9 15-2 9-0 15-7 12-7 
15-1 15-1 9-8 16-3 11-8 
15-5 15-2 9-0 16-6 12-4 
14-6 9-8 16-9 12-2 
14-9 9.4 15-7 11-8 
15-2 9-8 15-5 12-1 
15-8 9-1 16-4 12-4 
Mean — 15-2 | Mean — 15-1 Mean — 9-3 | Mean — 16-1 | Mean — 12-1 
top of the clock tower on the building 64 metres from the ground, gave 
a mean of.only 12-1 ions per c.c. per second. 
From the evidence adduced by C. 8. Wright* it would appear 
that the ice and water in Toronto Baycut off practically all the penetrat- 
ing radiation from the ground beneath. Assuming this to be so we obtain 
from the numbers given in Table II. the value 5-8 for the number of 
ions made per c.c. per second in the air in the receiver by the pene- 
trating radiation present at the lawn as well as by the secondary rays 
produced by this radiation at the metallic walls of the receiver and 
probably, too, at the surface of the ground. 
The table shows also, with the same assumption, that 2-8 represents 
the value for “q” arising from the penetrating radiation present at the 
top of the City Hall tower and from the secondary rays excited by it. 
These numbers shew that the effect of the penetrating radiation and its 
accompanying secondary radiation in ionizing the air in the receiver at 
the top of the tower was only about 48 per cent. of the effect produced 
by similar radiations on the University lawn. This diminution in 
intensity for an altitude of 64 metres is practically the same (as Table 
I. shews) as that calculated by Eve for a diminution of the earth’s 
penetrating radiation. 
* Wright, Phil. Mag. Feb. 1909, p. 310. 
