290 Mr. E. H. Nichols on the Diurnal 



Observations at Eskdalemuir during August 1914. 



As Kew Observatory is only a few miles from London 

 and practically at sea-level, it will be advantageous to 

 compare the electrical variation with that for Eskdalemuir 

 (237 metres above M.S.L.) in Dumfriesshire, removed from 

 disturbing artificial conditions. The importance of the elec- 

 trical investigation was increased by the fact that it was 

 possible to include a day of partial solar eclipse, and thus 

 compare the diurnal variation obtained with that for the 

 normal day. Observations of the conductivity variation were 

 made on six davs, and of the electric charges on seven days, 

 viz. August 13, 14, 17, 20, 22, 28, and 31. As the eclipse 

 day was August 21, it was considered important for compa- 

 rison to observe on the preceding and following day, and also 

 to have the other days well balanced with respect to August 21. 



Comparing the monthly means at Kew and Eskdalemuir, 

 the conductivity is nearly twice as great at the latter place 

 (see Tables I. and IV.). There is a corresponding early 

 morning minimum about 5 a.m. for both places, and the 

 July depression at Kew about midday is repeated for 

 August at Eskdalemuir. The most remarkable difference 

 in the variation is the sharp increase in conductivity at 

 Eskdalemuir between 8 and 10 a.m., there being only a small 

 rise at Kew. This is certainly to be associated with the 

 rapid disappearance at Eskdalemuir oE the morning mist, 

 which was frequently dissipated during these hours. 

 Because of the decreased relative humidity the charged 

 ions in the air would lose a percentage of their water 

 molecules, thus increasing their mobility and, consequently, 

 the conductivity of the air. 



It is noticeable that the charges at Eskdalemuir varied 

 less than at Kew. The mean positive charge is about 

 6 x 10 -18 e.m.u. at both places. For the electric charges 

 at Eskdalemuir there is a maximum about noon, and a 

 minimum during the night. There is also an indication of 

 a secondary maximum and minimum in the early morning. 



^— J shows a decrease during 



the day with a minimum at 1 P.M., which agrees well with 

 the Kew results. The air-earth current is somewhat irre- 

 gular, and there is no well-marked variation. It should be 

 noted that the conductivity and potential were both high at 

 Eskdalemuir, so that the air-earth current is high, while the 

 mobility shows 100 per cent, increase from 4 to 10 a.m. and 

 maintains an almost steady value for the rest of the day, 

 being about double that obtained at Kew. 



