62 ROYAL SOCIETY OF CANADA 
tion, and the variations which he did observe did not exceed 3% of the 
observed normal intensity, of the radiation. 
These observations were confirmed later by G. A. Cline and the 
conclusion was drawn by both these investigators that the soil contri- 
buted by far the greater proportion of the penetrating radiation present 
at the earth’s surface at Toronto, and that any portion which might be 
contributed by the earth’s atmosphere or by the sun was negligible in 
comparison with that arising from the soil. 
The view that the soil was the chief source of the radiation was 
corroborated later by the discovery of Prof. McLennan and C. S. Wright 
that the intensity of the radiation was very much less over the surface 
of Lake Ontario than over the surface of the land adjoining the waters 
of the lake. 
This diminution in the intensity of the radiation over lake waters 
was also observed later by Gockel,! at Vierwaldstattersee, in Switzer- 
land. 
From these experiments it seems then that there is no doubt about 
the soil and rocks of the earth being the chief source of the radiation. 
It appears too that the atmosphere of the earth contributes but a small 
proportion, if any, of the radiation. There is moreover, but little, if 
any, evidence to shew that any part of it has its origin in either the sun 
or the moon. 
Although it is clear from the evidence adduced above that it is not 
possible to ascribe an appreciable proportion of the pentrating radiation 
as ordinarily observed to a celestial origin, it seemed possible that the 
intensity of the radiation might be increased by the passage near to the 
earth of a body possessing the constitution and characteristics of Hal- 
ley’s Comet. At the suggestion of Prof. McLennan the writer, there- 
fore, undertook to make a series of observations on the intensity of the 
radiation extending over a number of days during the period when the 
comet and its tail was predicted to be in closest proximity to the earth. 
To make the observations an ionisation vessel, of about 30,000 ces 
capacity, similar to that used by C. 8. Wright, and later by G. A. Cline, 
was installed on the roof of the Physical Laboratory, and readings on 
the conductivity of the air in the vessel were taken with a sensitive 
quadrant electrometer situated in a room directly below the point 
where the ionisation vessel was installed. 
In setting up the apparatus the most extreme care was taken to see 
that all insulating parts were clean and dry, that the receiver itself was 
hermetically sealed and that all electrical connections were effectively 
and thoroughly screened. 

! Gockel, Phys. Zeit., Nov. 10, 1909, p. 846. 
