466 Professor Arthur Schuster [Feb. 22, 



could be obtained. These observations were made in clear weather. 

 The balloon afterwards passed over a layer of clouds, and strong electric 

 effects were noticed. Similar observations bad been previously made 

 by others (Andree, Le Cadet, and Bornstein), and though the subject 

 is by no means exhausted, we may take it as provisionally established 

 that the lines of force of the normal electric field of the earth end 

 within the first 10,000 feet or 15,000 feet. This result is of great 

 importance, for it shows that in fine weather there must be a layer of 

 positively electrified air permanently above us. Currents of air in 

 this layer must affect the field as we observe it, and possibly the 

 daily period may be due to changes in the currents of air at a 

 moderate height. A fact discovered by Exner is of importance in 

 connection with this subject. Observing at three different places (in 

 a field close to Vienna ; in St. Gil gen, on the Wolfgangsee ; and on 

 the hills near Venice), he found that whenever there was a strong 

 south wind, with a clear sky, the normal electric force was always 

 increased, and sometimes considerably.* 



The daily changes show, with few exceptions, a remarkable uni- 

 formity at different places. There are in general two maxima of poten- 

 tial — one at 8 or 9 o'clock in the morning, and one in the evening. 

 The evening maximum is the most marked, while at some places, and 

 especially near towns, the morning maximum disappears. The same 

 general features of the daily variation have been found to hold at a 

 number of European stations, at Cape Horn, Melbourne, and in the 

 Northern Arctic regions. If the variation is separated into two — 

 one having a period of 24 hours, and the other of 12 hours — the 

 latter is found to agree in phase at widely different places on the 

 earth's surface, while the former is found to vary to a much greater 

 extent, and hence to be probably more affected by local circumstances. 

 The remarkable researches of Hann have given a similar result for 

 the diurnal variations of the barometer, and we may reasonably con- 

 clude that the semidiurnal variation of atmospheric electricity is 

 connected with the same circulation in the upper regions of the atmo- 

 sphere which shows itself in the corresponding changes of pressure. 



In addition to the more regular periodic changes, the electric 

 stress observed in fine weather shows marked differences on different 

 days and at different seasons. With respect to these, the researches 

 of Prof. Franz Exner f have led to the important result that there is 

 a close connection, direct or indirect, between the amount of aqueous 

 vapour present in the atmosphere and the fall of potential observed 

 at the surface of the earth. If p be the pressure of aqueous vapour 

 present in centimetres, Exner 'deduces the equation for electric force P 



p- A 



1 + & Po ' 



where A = 1300, h = 13-1. 



* ' Wiener Akad. Sitzungsberichte,' vol. xcvi. 1887. f Ibid. 



