January 2, 1896] 



NATURE 



209 



others (Andree, Le Cadet, and Bornstein), ami though the 

 subject is by no means exhausted, we may take it as provision- 

 ally 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. 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, Sitzungsberichte, 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 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 conclude that the semidiurnal variation 

 of atmospheric electricity is connected with the same circulation 

 in the upper regions of the atmosphere 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 (ibid.) 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 />, be the pressure of aqueous vapour present in 

 centimetres, Exner deduces the equation for electric force P 



^= -At-' 



I +k/>o 

 where A = 1300, k = lyi. 



The formula agrees very well with observations in which the 

 vapour pressure varied between o'23 and 0'95, and it is 

 especially to be remarked that it is the amount of vapour and 

 not the humidity which determines the electric force. Observa- 

 tions made by Mr. E. Drory during a journey round the world 

 fit in very well with Exner's formula, and observations made at 

 such widely different places as Suez, Albany, Sydney, Colombo, 

 and Penang showed a fall of potential practically identical with 

 that calculated from the above formula, though the same 

 constants were taken and the vapour pressure varied between 

 0*8 cm. and 2 '2 cm. 



Messrs. Julius Elster and Hans Geitel (Wiener Akad. 

 Sitzungsberichte, vol, ci., 1892) have followed up this research. 

 Their investigations have shown a satisfactory agreement 

 with Exner's formula, if the mean values of a number 

 of observations in which the vapour pressure is approximately 

 the same, is considered. But individual numbers differ very 

 widely from the mean, so that the formula cannot be used to 

 predict the normal fall of potential on any particular day. 

 There is, perhaps, nothing surprising in the great divergence of 

 such individual results if it is considered that we only observe 

 the moisture near the surface of the earth, but are ignorant of 

 the total amount of water in the column of air over the district 

 in which the observations are carried out. The same authors 

 have shown that an equally good agreement can be obtained if, 

 instead of the amount of aqueous vapour, we take the intensity 

 of active radiation as the determining circumstance. The light 

 might be supposed to act on the general surface of the earth, as 

 it does according to Hallwachs' observations on a metallic body, 

 dissipating a regular charge. There are some difficulties in the 

 way of this explanation, the most serious being the absence of 

 experimental evidence that sunlight actually does act in the 



NO. 1366. VOL 53] 



manner indicated on any substance forming part of the earth's 

 surface. It is impossible at the present time to enter more fully 

 into this subject, but attention must be drawn to the very 

 important indirect result, that there seems to be a connection 

 between ultra-violet radiations and the amount of aqueous 

 vapour present in the air. 



The phenomena of atmospheric electricity have been studied 

 at the mountain observatory established on the " Sonnblick," in 

 Salzburg, at a height of 3100 metres. 



The inriportant result has been established that the electric 

 force is singularly constant. The great differences observed at 

 low level between the electric field in summer and winter, or on 

 dry and wet days, seems to be completely absent, and these facts 

 tend to support the conclusion derived from balloon observation, 

 that the positive ends of the lines of force are situated at a 

 height of something like 10,000 feet. 



Brief allusion must be made to some of the causes which 

 alter to a marked extent the normal fall of potential. As the 

 surface of the earth is negatively electrified, it follows that dust 

 carried up by the wind must be electrified, and it is found, 

 indeed, that in violent dust storms the laws of force near the 

 surfaces are altogether distorted and reversed in direction. 

 Werner Siemens (Pogg. Ann. cix., i860; Meteorologische 

 Zeitschrift, 1890, p. 252) could, while standing on the top of 

 one of the pyramids during a strong wind, charge an improvised 

 Leyden jar jsufficiently to obtain strong sparks. A casual 

 observation of Elster and (ieitel (Ziele mid Methodeu, p. 11) 

 may prove significant. On March 7, 1889, the temperature in 

 Wolfenbiittel was rising from - 10° C. to -f 2°C., a cirrus layer 

 covering the sky. The fall of potential changed in the course of 

 four hours from 1302 volts per metre to - 1200 volts, that is, 

 from a very exceptionally high fall to an equally strong gradient 

 in the other direction. Although the atmospheric circumstances 

 were anomalous, they seem in themselves not sufficient to account 

 for the anomalous electrical effects, and the authors suggest that a 

 possible explanation may be found in a violent dust storm which 

 on the previous day was observed in Alexandria. 



Fogs are generally found to increase the normal fall consider- 

 ably, so that the drops of water must be taken as positively 

 electrified. 



Waterfalls considerably disturb the electric condition of the air 

 in their neighbourhood, the air surrounding the fall being charged 

 negatively, sometimes to considerable distances. 



Whether clouds in themselves are always electrified is very 

 doubtful ; they no doubt disturb and generally weaken the fall 

 of potential at the earth's surface, but this may only be due to a 

 displacement of the positively-electrified layer which balloon 

 observations have shown to exist at a height of from 10,000 feet 

 to 20,000 feet. While a cloud discharges rain, the electrical 

 effects in the neighbourhood of the place are the same as that in 

 the neighbourhood of a waterfall. The explanation is probably 

 the same in the two cases, and by means of experiments, alluded 

 to further on, we may reproduce the negative electrification of 

 air under similar circumstances. 



Measurements of the electrification of falling rain or snow, 

 simple as they appear at first sight, are beset with very serious 

 difficulty. We owe the most complete investigation on the point 

 to Messrs. Elster and Geitel ( Wiener Sitzungsberichte, vol. xcix., 

 1890). They find no regularity in the electrification, though 

 positive signs slightly preponderate with snow and negative signs 

 with rjtin. 



The approach of a thunderstorm announces itself by charac- 

 teristic cumuli clouds, and the general atmospheric condition 

 favourable to their formation is felt by many persons of nervous 

 temperament. Many of us are accustomed to hear that " there is 

 thunder in the air." Whatever the special feeling of ' ' thunder " 

 may be due to, it cannot be an electrical effect, for electrical 

 instruments delicate enongh to detect a small fraction of the 

 normal force, give no indications of the approach of a thunder- 

 storm, and it is only when the cloud has begun to discharge 

 rain or hail that strong electrical effects are noticed. During 

 the thunderstorm the electroscope is, of course, much disturbed, 

 and there are frequent and violent reversals of its indications. ^ 

 The fact that no effects are observed at the surface of the earth 

 during the approach of a thunder-cloud does not prove that 

 there is no electrical separation, for we may imagine two 

 oppositely electric layers at different levels producing a strong 



i Weber, Elekirotechnische Zeitschrift, vol. x. ; Elster .-ind Geitel, 

 " Ueber einige Ziele und Methoden Luflelektrischer Untersuchungen," 

 Wolfenbuttel, 1891. 



