SECTION III., 1910. [61] Trans. R. $. C. 
VII.—On a Variation in the Intensity of the Penetrating Radiation at the 
Earth’s Surface Observed during the Passage of Halley’s Comet. 
By ARTHUR THOMSON, M.A. 
(Communicated by Professor McLennan and read Sept. 28, 1910.) 
Since the discovery in 1902 of the penetrating radiation which 
exists at the surface of the earth, a number of investigators have at- 
tempted to ascertain whether this radiation had its origin in the rocks 
and soil of the earth or in the atmosphere of the latter, or whether it 
had its origin in one or more of the extra terrestrial bodies such as the 
sun or the moon. 
A number of investigators including Borgmann,! Mach and Rim- 
mer,? Wood and Campbell, McKeon,‘ Strong,’ Pacini,® and Wulf,’ 
found in their various localities daily variations in the intensity of this 
radiation, and were able from their results to point to regular daily 
periods of maximum and minimum intensity. Further, an examination 
of the results of these investigators shews that the variation in the 
intensity of the radiation covers an exceedingly wide range. 
In some cases variations of 10% from the normal intensity were 
observed, but in the majority of cases variations very much greater than 
this amount were commonly noted. 
From a closer examination of the various results recorded, however, 
it does not appear that it is possible to establish any connection between 
the times of maxima and minima intensities obtained in different 
localities and as a consequence one is forced to the conclusion that the 
variations observed by the different observers were due to local causes 
and not to any influences arising from the proximity of the sun or any 
of the other celestial bodies. 
The results obtained by Prof. McLennan and his students at 
Toronto, when investigating this penetrating radiation, have been quite 
different from those obtained elsewhere, in that they failed to shew the 
marked variations observed by others. In particular C. 8. Wright, 
who made a number of observations at Toronto was not able to find any 
appreciable regular variation in the intensity of the penetrating radia- 


1 Borgmann, Se. Abs., 1905, No. 1580. 
2 Mach and Rimmer, Phys. Zeit., 7 pp. 617, Sept. 15, 1906. 
3 Phil. Mag., Feb. 1907. 
4 Phys. Rev., 1907. 
5 Phys. Rev., July 1908. 
6 Rend. Acc. Lincei, 18, 123-129, 1909. 
7 Phys. Zeit., 10, 1909, 152-157. 
