Electricity at the Kew Observatory. 113; 



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, even 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 r 

 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 elec- 

 tricity. I presume, however, that bright sunshine such as the 

 Campbell- Stokes instrument records always possesses this power, 



K 2 



