318 DR. C. CHREE: ATMOSPHERIC ELECTRIC POTENTIAL RESULTS AT KEW 
exerts, he says, at some stations a large effect in the early afternoon hours through 
absorbing the sun’s radiation of shortest wave-length. This absorbent stratum he 
believes especially characteristic of arid and dusty regions, while comparatively non¬ 
existent in moist climates, or where foliage is luxuriant. The conditions at Kew— 
a station situated in an extensive grass-covered park—are hardly such as Exner 
associates with a large 12-hour wave. It is possible, of course, that the large 
extension of London since 1864 may have profoundly altered the electrical conditions 
even as far west as Kew ; hut if so, one would exjDect an enormous difference to 
manifest itself when the direction of the wind changes from east to west. I am not 
prepared to say that such a change of direction is wholly without effect—as it is 
difficult to distinguish between the direct effect of change of wind direction and the 
indirect effect of the change of weather conditions accompanying it—but if it exists it 
is by no means conspicuous. Also the difference between the earlier and later Kew 
data is not an increase in the 12-hour wave, but a diminution in the 24-hour wave. 
Faulty conditions or local peculiarities seem a priori more likely to introduce or 
enhance fictitious waves than to wipe out real periodic terms. The observed 24-hour 
wave, it must be remembered, shows a phase angle which varies largely with the 
season of the year, but the differences between the phase angles observed in 1862-4 
and 1898-1904 are in most months comparatively small. If, then, the reduction in the 
amplitude apparent in the later series is due to some disturbing cause, that cause 
must have itself a phase which nearly keeps step with that of the real 24-hour 
potential wave throughout the year. 
Before attaching much importance to views which after all are mainly theoretical, 
it should be remembered that the older Kew data refer to all days, the later only to 
fair weather conditions. Future investigations may show the 24-hour term to be 
more dependent on the meteorological conditions than the 12-hour term. Also in 
considering the relative importance of the two terms we should make allowance for 
the fact that it is the conditions close to the ground that are at present of most 
immediate practical importance both to human beings and to agriculture. 
§ 15. Returning to the consideration of Tables VIII. and IX., it will be seen that 
with the exception of the values of a 4 in winter—when the amplitude at Kew was 
so small that considerable uncertainty must exist—the Kew and Bureau Central 
values of a 3 and a 4 are sufficiently close to raise a strong presumption that the 8- and 
6-hour terms represent a true atmospheric electricity effect. The comparative 
agreement between the phase angles from the earlier and later series of observations 
at the Bureau Central and the Eiffel Tower favour the same conclusion. 
The 8- and 6-hour waves are decidedly smaller at Kew than at the Bureau Central. 
The difference in this case may not improbably be partly due to greater homogeneity 
in the Kew data. The fact that the 8- and 6-hour terms are also much less at the 
Eiffel Tower than at the Bureau Central must, however, have some other explanation; 
it is most natural to ascribe it to the influence of height. 
