96 ATMOSPHERIC ELECTRICITY. 



is greater at a height of, say, 200 meters than on the surface of the 

 earth. The observations of Br. Leouhard Weber (Elektrotecknische 

 Zeitschrift, April, 1888) bring out this point clearly. In one case the 

 fall of potential at a height of 350 meters was found to be six times 

 that at the earth's level. This increase is in itself not surprising, if 

 we remember that every particle of dust raised from the ground must 

 itself be negatively electrified, and probably the observed increase in 

 the electric force is sufficiently accounted for by the presence of elec- 

 trified dust. 



Observations made at greater heights in balloons, on the other hand, 

 seem clearly to indicate that this increase soon ceases, and that a 

 diminution already takes place at moderate heights. Thus the obser- 

 vations of Dr. O. Baschin (Meteorologische Zeitschrift, September, 

 1894) gave for the fall of potential in volts per meter the numbers 49, 

 28, 13 at heights of 760, 2,400, 2,800 meters respectively, and at a height 

 of 3,000 meters no measurable fall at all 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 had 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 differ- 

 ent places (in a field close to Vienna; in St. Gilgen, 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 (Wiener Akad. Sitzungsberickte, 

 Vol. XCVI, 1887). 



The daily changes show, with few exceptions, a remarkable uniformity 

 at different places. There are in general two maxima of potential — 

 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 espe- 

 cially near towns, the morning maximum disappears. The same gen- 

 eral 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 twenty-four hours and the other of twelve hours — the latter 

 is found to agree in phase at widely different places on the earth's sur- 

 face, 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 Harm have given a similar result for the 



