ATMOSPHERIC ELECTRICITY. 97 



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 

 vapor present in the atmosphere and the fall of potential observed at 

 the surface of the earth. If p be the pressure of aqueous vapor pres- 

 ent in centimeters, Exner deduces the equation for electric force P 



1 + lcp 

 where A = 1300, Jc = 13.1. 



The formula agrees very well with observations in which the vapor 

 pressure varied between 0.23 and 0.95, and it is especially to be 

 remarked that it is the amount of vapor and not the humidity which 

 determines the electric force. Observations 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 vapor pressure varied between 0.8 cm. 

 and 2.2 cm. 



Messrs. Julius Elster and Hans Geitel (Wiener Akad. Sitzungs- 

 berichte, Vol. CI, 1892) have followed up this research. Their investi- 

 gations have shown a satisfactory agreement with Exner's formula, if 

 the mean values of a number of observations in which the vapor pres- 

 sure is approximately the same are considered. But individual num- 

 bers differ very widely from the mean, so that the formula can not 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 vapor, 

 we take the intensity of active radiation as the determining circum- 

 stance. 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 experi- 

 mental evidence that sunlight actually does act in the 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 

 sm 95 7 



