132 Observations on Atmospheric Electricity at Kew. 



Table XXIII. 



Potential at Station A. 

 Series of 

 observations. 

 I. 

 II. 

 III. 

 IV. 



The density of aqueous vapour is a quantity having but a small 

 diurnal variation,* and it would appear, from a table published by 

 General Sabinef that the calculated mean potential for the day 

 taken as the mean of the calculated values for the 24 hours would 

 differ but little from that answering to only the two times, 10.30 A.M. 

 and 4.30 P.M. Thus the calculated values in Table XXIII may be 

 regarded as close approximations to the true calculated means for the 

 seasons of the four observations. On the other hand, according to the 

 table of diurnal variation of potential in the paper by Mr. Whipple 

 already referred to, the true means obtained from observations at 

 every hour of the day might be expected to be on an average some 

 10 per cent, higher than the observed values in Table XXIII. It 

 ought, further, to be remembered that, as explained in 4, the 

 potential at station A must fall short of the true potential at a point 

 60 inches above the ground in the open, also the fraction of the 

 existing potential picked up by the portable electrometer may be 

 appreciably less than unity. Thus the fact that the calculated values 

 in Table XXIII are so decidedly larger on the average than the 

 observed is perhaps rather in favour of the formula than otherwise. 



If we may judge, however, from the few data in the table, there 

 seems some ground for the suspicion that the formula will prove to 

 give too narrow a range. 



Before concluding, I have much pleasure in acknowledging the 

 ready and valuable help I have received from Mr. E. Gr. Constable, 

 Senior Assistant at the Kew Observatory. Mr. Constable took all 

 the electrical observations and the measurements of the meteoro- 

 logical curves, and gave me in addition much useful information 

 derived from his long experience of the working of the electrograph 

 and portable electrometer. 



* A fact difficult to reconcile with the general form of Exner's theory, 

 f ' Roy. Soc. Proc.,' vol. 18, 1869, p. 8. 



