Page:Proceedings of the Royal Society of London Vol 60.djvu/129

114 though doubtless in very variable degrees at different seasons. Solar radiation occurring after an observation is taken, clearly cannot affect it. Thus the data got out as to the amount of bright sunshine recorded prior to the observations must, I think, bear fairly directly on Elster and Geitel’s theory. If it be true, the potential gradient must, I think, fall conspicuously as the number of hours of previous sunshine increases.

§ 19. An objection of a different kind is the proximity of the Kew Observatory to London. This objection has already been urged against Greenwich by investigators* whose theories do not harmonise with the results obtained there. A weekly period exists, they say, in the Greenwich electrograph curves, and this, they assume, can arise only from a weekly fluctuation in the amount of smoke, due to our insular habits of keeping Sunday. If, for a moment, we suppose the phenomenon and explanation both true—a pretty large assumption—there seems a wide step to the conclusion that results so affected are useless. I do not myself see that they need lead to erroneous conclusions, unless one is dealing with a cycle whose period is seven days, or a multiple thereof, which a lunation, for instance, is not.

In the present instance I would point out that the prevailing winds during each one of the series of observations were from directions included between N.N.W. and S., and that as Kew Observatory is some miles to the ivest of London, while the manufacturing districts are mainly in the east, it is difficult to see how London smoke could affect the results. The Observatory, I should add, is situated: in a large open park to the immediate west of tbe extensive Kew Gardens.

Even if the prevailing winds had been easterly, T question whether smoke would have exerted an appreciable influence. The analysis above mentioned of the electrograph results for 1880, by the late Mr. Whipple, seems to show that if any relation existed then between electric potential and wind direction, it varied with the season of the year; this would hardly have occurred if smoke present in east winds had an appreciable effect. Tables of Results.

§ 20. In discussing the observations, I have decided to commence by incorporating the actual details in a series of tables. This will enable any one to judge for himself whether the conclusions finally arrived at are in accordance with the facts. The first eight tables give full particulars of the results. The arrangement is not

k. Svenska Vet.-Akad. Handlingar,’ Band 19 Afd. 1, No. 8, Stockholm, 1894.
 * gee pp. 42—43 of offprint of paper by Ekholm and Arrhenius in ‘Bihang till