106 Dr. C. Chree. Observations on Atmospheric 



doubtless due in great part to changes occurring whilst the observa- 

 tions were in progress. 



Possible Influence of Pattern of Instrument. 



10. The conclusions in the previous paragraph refer as yet only 

 to the portable electrometer. They can be extended to the electro- 

 graph records only if we are able to show that a fairly uniform 

 ratio exists between the potential obtained with the water-dropper 

 at a fixed station and that obtained with the portable electrometer at 

 one or other of the stations A to E. 



The position of the water-dropper was maintained undisturbed, 

 barring accidents, throughout the observations. It thus suffices to 

 compare the curve readings with the corresponding ones with the 

 portable electrometer at station A. The curves were accordingly 

 measured at the mean times of each set of observations. The ratios 

 of the individual readings to those at station A were calculated, 

 and results obtained analogous to those in Table II. It will suffice 

 for our present object to consider the results analogous to those in 

 Table III. 



Table VII. 



Percentage Deviations from the Means (Electrograph/Portable). 



Series of observations. 



*!. II. III. IT. 

 Percentage deviations 28 30 35 28 



The spot where the jet breaks up resembles B more closely than 

 any other station, and shares its low potential. Further, the electro- 

 graph curves are read to the nearest 5 volts only, so that uncertainties 

 in the reading are even more important than with the portable, read to 

 the nearest 1 or 2 volts, at station B. Thus, the results in Table VII 

 are, at least, not conspicuously worse than those in Table III. As a 

 matter of fact, the results in Table VII were, I believe, somewhat 

 prejudiced by a variation in the water jet throughout the day (see 

 11). Supposing this defect removed, the evidence points to the 

 conclusion that the diurnal, and possibly the annual, variations got 

 out with the water-dropper situated in the Observatory, and the 

 portable electrometer at station A, may be expected to be in good 

 accord, assuming the conditions under which each instrument works 

 to be maintained uniform. 



Attention was also directed to the possibility of the two different 

 patterns of instrument being differently affected by the same climatic 

 conditions. Each series of observations the forenoon and afternoon 

 observations of series' II and III being treated separately was 



