ability for comparison with other results free from these extraneous effects.
As to intrinsic value, there are, at least in England, seasons of the year when a nearly cloudless day is exceptional. For instance, during November and December, 1894, in ten days out of eighteen, on which observations were taken a little before noon, no bright sunshine was recorded up to the hour of observation. At such a season, if one confined one’s attention to nearly cloudless days, hardly any data would be obtained, and they might not unreasonably be regarded as abnormal.
As to the disturbing action of clouds, this is no doubt in some cases very large; but with clouds of this character the influence may be considerable when they cover only a small fraction of the sky, and probably, in some cases, eveu when they are below the horizon. Thus on one occasion at Kew, when part of the sky was covered by a thundercloud—so distant that only one or two faint lightning flashes were detected—sudden changes of potential of thousands of volts from negative to positive and back again were observed on the roof, whilst the sun shone at intervals. The sudden alternations of potential doubtless accompanied flashes of lightning, but no rain was falling anywhere near, and possibly an observer a few miles away might have regarded the day as an ideal quiet one. Again, there are other forms of clouds whose influence seems not unlikely to be much less than that of invisible vapour in motion nearer the ground. The mere interception of sunlight by cirrus clouds or detached masses of cumulus, if we may judge from some few experiments at Kew, has little if any effect.
It should also be borne in mind that wind velocity and amount of cloud must both have varied appreciably from day to day, and even throughout the individual days of Exner’s experiments. Some one —I forget who—defined a “ quiet ” day as one in which the flame of Exner’s electrometer was not blown out. All the days of the Kew observations satisfied, of course, this definition, if one is allowed to substitute the portable electrometer for Exner’s, yet on one occasion the anemometer was recording a mean velocity of forty miles an hour.
If aqueous vapour, as Exner supposes, is the sole, or even the dominant, agent in producing changes in potential, its activity can hardly be confined to days when there is little cloud, and the wind is low.
§ 18. As regards Elster and Geitel’s theory, the data available for criticism are, I admit, defective, inasmuch as no measurements are taken at Kew of the dissipative effect of sunlight on negative electricity. I presume, however, that bright sunshine—such as the Campbell-Stokes instrument records—always possesses this power,
