318 ATMOSPHERIC ELECTRICITY. 



and as it is difficult to conceive of any meteorological condition which is the same during 

 calms and winds and during summer and winter, we are forced to the conclusion that the 

 variations in the potential gradient are real and not due to local meteorological condi- 

 tions. 



It has already been remarked that the daily variation of the potential gradient found 

 in McMurdo Sound is very abnormal. It was also shown above that the yearly variation in 

 the Antarctic app3ars to bs different from that of the rest of the world ; are the abnormal 

 yearly and daily variations in tho Antarctic related ? The only other observations of the daily 

 variation of the potential gradient made in the Antarctic (other than those made by Bernacchi 

 in McMurdo Sound) are two series made by members of Charcot's two expeditions on the 

 coast of Graham's Land.* Both of these show a variation very similar to the normal variation 

 found in other parts of the world: the main minimum is near tod a.m., but the main 

 maximum is early, occurring at 3 p.m. instead of at 8 p.m. as is more usual. 



We are thus led to conclude that the daily variation of the potential gradient in McMurdo 

 Sound is not only different from that in other parts of the world but is different from that of other 

 places in the Antarctic. In other words not only is the daily variation abnormal but so far as 

 observations go it is unique. The question at once arises is there anything in the geographical 

 position of McMurdo Sound which also is miique ? McMurdo Sound possesses one feature which is 

 different from all other places for which atmospheric electricity obs3rvations are available. On 

 account of its position batween the magnetic and the geographical poles the direction of the 

 earth's magnetic field relative to the direction of rotation of the earth is practically reversed 

 from what it is at all other stations. | In other words in this part of the world if an 

 observer stands with his face looking along the lines of magnetic force he moves on account 

 of the earth's rotation from right to left instead of from left to right. In view of our total 

 ignorance of any relationship between terrestrial magnetism and atmospheric electricity it is 

 impossible to say what effect one would expect from this reversal, but a comparison between 

 the daily variation of the potential gradient in McMurdo Sound with that of the most 

 northerly station for which we have a complete set of data is instructive and sugges- 

 tive. 



During 1903-04 the present writer spent fourteen months in the Lai:)p village of Karasjok 

 (69° 17' N., 25° 35' E., 129 metres above sea-level) with the object of investigating the 

 electrical conditions of the atmosphere there.J The results of the daily variation of the 

 potential gradient for a whole year are shown in the upper curve of figure 93. As the 

 amplitude of the daily variation in Karasjok was much larger than in McMurdo Soimd, the 

 scale has been reduced in the ratio of 25=^10. A comparison between the curve for Karasjok 

 and the curve for Caps Evans given in figure 92 shows a great similarity in the general 

 shape of the two curves, but the times at which the maxima and minima occur are different. 

 The chief minimum occurs at 14| hours at Cape Evans and at 5 hours at Karasjok, 

 a difference of 9J hours ; while the chief maximum occurs at 7J hours at Cape Evans 

 and at 21 hours at Karasjok, a difference of 10^ hours. It is obvious, therefore, that if the 

 time at Cape Evans were put back 10 hours, the curves would agree not only in shape but 

 also in phase. To bring this out more clearly the curve for Cape Evans has been smoothed 

 by the formula — ^ — and plotted under the curve for Karasjok with its time values 

 put back 10 hours. The similarity of the two curves is now very apparent. 



* I. I. Key. E.xpeditioa antaiotique francjaise, 1903-05. Hydrographic, Physique du Globe, Paris, 1911, page 191. 



I. Rouch. Deu.xiome Expedition antarotique fran(;ai3e, 1908-10. Sciences physiques, Paris, 1913, page 14. 

 t The declination at Cape Evans is 153^ E. 

 t G. C. Simpson. Philosophical Transactions, A, 205, pagea 4-97, 1905. 



