Section III., 1909. [I i 1] Trans. R. S. C. 



VIII. — On the Penetrating Radiation at the Surface of the Earth. 

 By G. A. Cline, B.A. 



(Communicated by Professor McLennan, and read before the Royal Society 

 of Canada on May 26th, 1909.) 



I.— INTRODUCTION. 



During the last few years a number of investigators have made a 

 study of the penetrating radiation which is known to be present at the 

 surface of the earth with a view to locating its source or origin. The 

 presence of radioactive substances in the soil and of radioactive emana- 

 tions in the atmosphere suggest both the earth and the atmosphere as 

 sources for part at least of this radiation. It is possible, too, for such 

 a penetrating radiation as that present at the earth to have its origin 

 in the sun or other celestial bodies. 



It is known, besides, that the amount of radioactive emanation 

 present in the atmosphere at any time is largely dependent upon the 

 barometric changes which have taken place for a short time previously, 

 and upon the precipitation which has occurred in the locality concerned. 

 If then the penetrating radiation comes largely from the atmosphere it 

 would follow from the above considerations that frequent though 

 perhaps irregular changes should occur in the intensity of the radiation 

 at any particular locality. 



If the sun, however, contributed the major portion of the pene- 

 trating radiation we should then expect to find regular daily variations 

 in its intensity. On the other hand, if the greater part of the pene- 

 trating radiation has its origin in radioactive substances in the soil and 

 rocks, we should expect to find but little, if any, variation in its inten- 

 sity throughout the day or even from day to day in any particular 

 region. 



One of the first to note a diurnal change in the ionisation of air 

 contained in closed metallic cylinders was J. J. Borgmann.' His 

 experiments were carried out in the neighbourhood of St. Petersburg. 

 Among other results he found that the ionisation of the air in cylinders 

 of copper, aluminium, brass, iron, zinc, tin, or lead, increased for some 

 days after being enclosed, and finally reached a limit. This limit he 

 found to vary from day to day and even during a single day. The 

 minimum of ionisation noted by him was most frequently observed 

 ■about three o'clock p.m. 



^ Science Abstracts, 1905, No. 1580. 



