314 



ATMOSPHERIC ELECTEICITY. 



Table 158. 

 Daily variation of potential gradient. 

 Fine weather and wind — 5 miles an hour. 



Tbe curves based on this table are the thick ones shown in the upper part of figure 92. 

 It will be seen at once that the daily variation is very similar in all seasons, the maximum 

 occurring during the forenoon and the minimum during the afternoon. From the few observa- 

 tions made by Bsrnacchi on Captain Scott's first expedition C. T. R. Wilson drew the same 

 conclusion, so there can fie little doubt that this is the normal course of the potential 

 gradient in McMurdo Sound. It is a surprising result for it is very abnormal. At all other 

 places in the world where reliable observations of the potential gradient have been made the 

 main minima and maxima occur near 4 a.m. and 8 p.m. respectively. 



The question at once arises is the abnormal daily course of the potential gradient in 

 McMurdo Sound due to the local meteorological conditions or is it a true electrical effect ? 



If the two last columns of table 158 are compared, it will be seen that there is a re- 

 markable parallelism between the value of the potential gradient and the number of hours 



