Electricity at the Kew Observatory. 107 



arranged in descending order of some one meteorological element. Sup- 

 pose there to be 2n or 2n + I observations in the series (or half series) 

 the mean values of the ratios borne by the electrograph readings to 

 the corresponding ones with the portable electrometer at station A 

 were calculated for the first n and the last n instances separately. 

 Supposing r 1} r 2 , and r to denote the mean ratios for the first n, the 

 last n, and the whole 2n (or 2n + l) observations, then 



i{0-i-r a )/r}xioo 



may be regarded as the average percentage deviation of the two 

 groups from the mean. Table VIII gives the value of this quantity 

 in the case of the three meteorological elements from which a differen- 

 tial effect was most feared. 



Table VII T. 

 Value of M0-i-r 2 )/r} x 100. 





A plus sign denotes that when the meteorological element in 

 question was above its mean the water-dropper was more than usually 

 effective, relative to the portable electrometer ; a minus sign implies 

 the contrary. 



The evidence in the case of wind velocity is so contradictory that 

 we can safely assume that no uniform differential action exists. 



In the case of the two other elements the evidence is more consistent, 

 and it is possible that a small differential action may exist. It looks 

 as if much moisture, when not counterbalanced by a contrary action of 

 sunshine, tends slightly to pull down the reading of the water- 

 dropper relatively to that of the portable electrometer. The pheno- 

 menon, supposing it to exist, might be ascribed to a loss of efficiency 

 in a water jet when the vapour in the air increases, and a similar loss 

 in a flame collector during bright sunshine. But an influence at least 

 as likely is that of moisture, during damp weather, on the insulation 

 of the electrograph. 



