[CLINE] RADIATION AT THE SURFACE OF THE EARTH 113 



Strong from his measurements drew the conclusion that the 

 atmosphere contributed by far the major portion of the penetrating 

 radiation. He found the intensity of the radiation greater in summer 

 than in winter, and he too noted a double diurnal period in the ionisa- 

 tion in his electroscope. Precipitation of rain or snow always produced 

 a drop in the intensity of the radiation. But all changes in intensity 

 were eliminated when he surrounded his electroscope with thick lead 

 and iron screens. Strong's measurements are rather remarkable for 

 the extremely wide variations which they indicated and it is difficult 

 to account for them. On Jan. 30, 1907, for example, he observed 

 ionisations at various times of the day represented by 12, 82, 100, 77 

 on an arbitrary scale, and again on Sept. 3, 1909, ionisations were 

 observed represented by 25, 18, 50 and 155, 150, 42, 10, 15 on the same 

 scale. Variations so extensive as these do not appear to have been 

 observed by any other investigators and they seem to point to some 

 very special and exceptional local conditions. 



Some experiments were also made recently on this phenomenon 

 by D. Pacini at Sestola, in Italy.' This investigator used an alumin- 

 ium-leaf electroscope and studied the ionisation in air enclosed in 

 large zinc receivers. He too found daily maxima and minima values 

 in the ionisation. His minimum observations ranged from 8, 9, 10 to 

 12 ions per cc. per second while his maximum observations extended 

 in some cases to as high as 30 ions per cc. per second. His ionisation 

 values present a double daily period with two maxima at two to three 

 o'clock and nine to ten o'clock, and two minima from seven to eight 

 o'clock and from twelve to one o'clock. 



Wulf," too, who devised a new type of electrometer the movable 

 system of which consists of a double conducting quartz thread, has 

 applied this instrument also to the investigation of the penetrating 

 radiation. His experiments were carried out at Valkenburg in Holland, 

 both on the surface of the earth and in the chalk quarries in the 

 neighbourhood of that municipality. He too found a parallelism 

 between the intensity of the penetrating radiation and that of the 

 atmospheric potential, maximum values being obtained for both 

 phenomena in the summer about eight or nine in the morning and 

 evening, and minima about noon and midnight. In winter the midday 

 minimum was only slightly marked. 



The amplitude of the morning variations was found to be about 

 16%, while that of the afternoon ones was about 10% of the total 

 ionisation and the mean ionisation corresponded to from 25 to 30 ions 

 per cc. per second. The ionisations observed in the measurements in 



' Rend. Ace. Lincei, 18, 123-129, 1009. 

 ^Phys. Zeit. 10, 1909, 152-157. 



