ATMOSPHERIC ELECTRICITY IN HIGH LATITUDES. 89 



The effect of the clouds is not easy to understand, and as the results do not show 

 a pronounced dependence of the radio-activity on the clouds, perhaps it is not real, 

 unless it is that clouds are usually associated with ascending currents of air. Other 

 observers* have found a relation between radio-activity and clouds, but until more 

 observations are made the question must be left unsettled. 



Thus we see that the whole effect of the meteorological conditions on the radio- 

 activity depends on whether those conditions tend to mix the emanation rapidly with 

 a large volume of air, or to keep it near to the ground from which it is always 

 escaping. 



The same principles lead us to an explanation of the daily and yearly course of 

 the radio-activity. During the daytime ascending currents are formed, while as the 

 evening approaches these stop and the air lies cold and stagnant during the night. 

 Thus we see why the minimum should be in the daytime and the maximum during 

 the night. 



The yearly period has a similar explanation. During the winter in Karasjok, when 

 the snow is permanently on the ground, temperature inversion accompanied by 

 stagnant air is the rule rather than the exception. On the contrary, during the 

 summer when there is nearly permanent sunshine, ascending currents will be formed 

 at all times of the day and night. This accounts for the winter maximum and the 

 summer minimum of the radio-activity. 



One strange fact is that the activity should be so high during the winter when the 

 whole country is covered with snow. This at first led me,t with other observers,^ to 

 doubt ELSTER and GEITEL'S theory, but the reason is not hard to find if it is 

 remembered that the snow must form a very large reservoir to hold the emanation as 

 it is escaping from the soil. It would be interesting to see if air, drawn from the 

 snow in the way ELSTER and GEITEL drew it from the ground, would be charged 

 with emanation. I wished to test this, but had no instruments with me which could 

 be used for the experiment. 



One would also expect high values of the radio-activity in Karasjok during the 

 winter from another consideration. Karasjok is situated on the river, and just as the 

 water from all the surrounding land flows down to the river, so when the temperature 

 falls very low the cold air will also find its way into the river valley. This cold 

 air flowing over the ground will sweep the emanation along with it, and so the valley 

 will become filled with air highly charged with emanation. 



In order to find if the minerals of Karasjok are particularly rich in radio-active 

 constituents I sent samples of sand and rock to the Hon. R. J. STRUTT, who very 

 kindly undertook to test them, and to whom my best thanks are due for the trouble 

 he took in his investigation of them. In none of the specimens was he " able to 



* GOCKEL, ' Phys. Zeit.' ; ZOLSS, ' Phys. Zeit.' 

 t ' Roy. Soc. Proc.,' vol. 73, p. 209, 1904. 

 I ' Phys. Zeit.,' 5, p. 591, 1904. 

 VOL. CCV. A. N 



