Page:Proceedings of the Royal Society of London Vol 60.djvu/121

106 doubtless due in great part to changes occurring whilst the observations were in progress.

§ 10. The conclusions in the previous paragraph refer as yet only to the portable electrometer. They can be extended to the electrograph 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.

Percentage Deviations from the Means (Electrograph/Portable). Series of observations.

I. II. III. IY. 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 electrograph 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 YII 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