98 ATMOSPHERIC ELECTRICITY. 



must be drawn to the very important indirect result, that there seems 

 \o be a connection between ultra-violet radiations and the amount of 

 aqueous vapor 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 3,100 meters. 



The important 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, 1860; 

 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 sufficiently to obtain strong sparks. A casual observation 

 of Elster and Geitel (Ziele und Methoden, p. 11) may prove significant. 

 On March 7, 1889, the temperature in Wolfenbiittel was rising from— 10° 

 C. to + 2° C, a cirrus layer covering the sky. The fall of potential 

 changed in the course of four hours from 1,302 volts per meter to —1,200 

 volts — that is, from a very exceptionally high fall to an equally strong 

 gradient in the other direction. Although the atmospheric circum- 

 stances 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 considerably, so 

 that the drops of water must be taken as positively electrified. 



Waterfalls considerably disturb the electric condition of the air in 

 their neighborhood, the air surrounding the fall being charged nega- 

 tively, 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" a posi- 

 tively-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 neighborhood of the place are the 

 same as that in the neighborhood of a waterfall. The explanation is 

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

 alluded to farther on, we may reproduce the negative electrification of 

 air under similar circumstances. 



