468 Professor Arthur Schuster [Feb. 22, 



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 * 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,f may prove significant. On 

 March 7th, 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 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 not in them- 

 selves 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 in their 

 neighbourhood, the air surrounding the fall being found 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 dis- 

 placement of the positively-electrified layer which balloon observa- 

 tions 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 neighbour- 

 hood 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.J They find no regularity in the electrification, 

 though positive signs slightly preponderate with snow and negative 

 signs with rain. 



The approach of a thunderstorm announces itself by characteristic 

 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 



* 'Pogg. Ann.,' cix. 1860; « Meteorologische Zeitschrift,' 1890, p. 252. 



t 'Ziele und Methoden,' p. 11. 



X 'Wiener Sitzungsberichte,' vol. xcix. 1890. 



