590 



SCIENCE. 



[N. S. Vol. I. No. 22. 



electrified masses of aii- moving past the 

 place of observation. I did not remark 

 then, but I now see, that the electricitj^ in 

 these moving masses of air must, in all 

 probability, have been chiefly positive to 

 cause the variations which I observed, as I 

 shall explain to you a little later. 



§ 2 . Soon after that time a recording at- 

 mospheric electrometer * which I devised, 

 to show by a photographic curve the con- 

 tinuous variation of electric potential at a 

 fixed point, was established at the Kew 

 Meteorological Observatory, and has been 

 kept in regular action fi-om the commence- 

 ment of the year 1861 till the present time. 

 It showed incessant variations quite of the 

 same character, though not often as large, 

 as those which I had observed on the sea- 

 beach of Arran. 



Through the kindness of, the Astronomer 

 Eoyal, I am able to place before you this 

 evening the photographic curves for the 

 year 1893, produced by a similar recording 

 electrometer which has been in action for 

 many years at the Eoyal Observatory, 

 Geeenwich. They show, as you see, not 

 infrequently, during several hours of the 

 day or night, negative potential and rapid 

 transitions from large positive to large 

 negative. Those were certainly times of 

 broken weather, -wTith at least showers of 

 rain, or snow, or hail. But throughout a 

 very large proportion of the whole time the 

 curve quite answers to the description of 

 what I observed on the Arran sea-beach 

 thirty-six years ago, except that the varia- 

 tions which it shows are not often of so 

 large amount in proportion to the mean or 

 to the minimums. 



§ 3. Thinking over the subject now, we 

 see that the gradual variations, minute af- 

 ter minute through so wide a range as the 

 3 or 4 to 1, which I frequently observed, 

 and not infrequently rising to twenty times 

 the ordinary minimum, must have been due 



* 'Electrostatics and Maanetism.' 



to 2}ositively electrified masses of air, within 

 a few hundred feet of the place of observa- 

 tion, wafted along with the gentle winds of 

 5 or 10 or 15 feet per second which were 

 blowing at the time. If any comparably 

 large quantities of negatively electrified air 

 had been similarly carried past, it is quite 

 certain that the minimum observed poten- 

 tial, instead of being in every ease positive, 

 would have been frequently large negative. 



§ 4. Two fundamental questions in re- 

 spect to the atmospheric electricity of fair 

 weather force themselves on our attention: — 

 (1) What is the cause of the prevalent posi- 

 tive potential in the air near the earth, the 

 earth's potential being called zero? (2) 

 How comes the lower aii" to be electrified 

 to different electric densities whether posi- 

 tive or negative in different parts ? Obser- 

 vations and laboratory experiments made 

 within the last six or eight years, and par- 

 ticularly two remarkable discoveries made 

 by Lenard, which I am going to describe to 

 you, have contributed largely to answering 

 the second of these questions. 



§ 5. In an article 'On the Electrification 

 of Ail- by a Water-jet,' by Magnus Mac- 

 lean and Makita Goto,* experiments were 

 described showing air to be negatively elec- 

 trified by a jet of water shot vertically 

 down through it fi-om a fine nozzle into a 

 basin of water about 60 centimeters below it. 

 It seemed natural to suppose that the ob- 

 served electrification was produced bj- the 

 rush of the fine drops through the air ; but 

 Lenard conclusively proved, by elaborate 

 and searching experiments, that it was in 

 reality due chiefly, if not wholly, to the 

 violent commotions of the drops impinging 

 on the water surface of the receiving basin, 

 and he found that the negative electrifica- 

 tion of the air was greater when thej' were 

 allowed to fall on a hard slab of any material 

 thoroughly wetted by water than when they 

 fell on a yielding surface of water several 



*Philos02)Jiical Magazbie, 1890, second half-year. 



