AT KEW OBSERVATORY, 1898 TO 1912. 



157 



In the case of c 2 the reverse holds. The phase angles from the two periods show a 

 closer agreement than the amplitudes. The most natural inference is that even 15 

 years is too short a period to give a wholly normal annual variation for i\ and c 2 . This 

 conclusion is supported by fig. 4. The c and c 2 curves shown there, while much 

 smoother than the corresponding curves in E l5 present some features which- suggest 

 abnormality, notably as regards the points representing the September values. 



TABLE X. Annual Variation. Amplitudes and Phase Angles. 



15. In a recent interesting paper* dealing with atmospheric electricity data 

 obtained at Edinburgh during 1912, Messrs. CAUSE and SHKAHEII compare a good 

 many of their potential gradient results with those given for Kew in E,. Apart 

 altogether from the question of the absolute value of the potentials given in E 1; some 

 doubt may be felt as to how far the results serve to compare the two stations. 

 Messrs. CAUSE and SHEARER employed all the days whose trace was uninterrupted and 

 free from insulation troubles. As these numbered 302, in a single year, a considerable 

 proportion presumably were days when rain fell or negative potential occurred. 1 am 

 unable to say definitely how results derived from 302 days in one year at Kew 

 would compare with the results from the 120 selected rainless days. But a recent 

 paper by Mr. GORDON DoBSON,t which amongst other matters compares results from 

 different species of days during 1911 at Eskdalemuir, throws some light on the 

 question. Mr. DOBSON gives three series of diurnal variation results, the first derived 

 from what are known as days 101 in number whose characteristics fairly 

 correspond with those of quiet days at Kew; the second from all ordinary days, i.e., 

 days when the record was complete and conspicuously irregular variations were absent ; 

 the third from all hourly readings irrespective of whether the day's record was 

 complete or not. 



The ordinary days which included the O a days numbered only 155, so that the 

 difference between them and the O a days was very probably less than the difference 

 one would find between all complete days at Kew and the selected quiet days. The 

 mean potential gradients for the year derived from the three series of Eskdalemuir 

 data were respectively 234, 219 and 185. For the four midwinter months, when O a 



* 'Proceedings Royal Society of Edinburgh,' vol. 33, 1913, p. 317. 

 t 'Meteorological Office Geophysical Memoirs,' No. 7, London, 1914. 



