2 Dr. G. C. Simpson on the 



B. Thunderstorm precipitation. 



(4) The precipitation is sometimes positively and sometimes 



negatively charged. 



(5) More positive than negative electricity is brought down 



by the precipitation. 



(6) The charges per unit mass of the precipitation and the 



vertical electrical currents produced by its fall are 

 much larger than with non-thunderstorm rain. 



(7) The potential gradient undergoes large and rapid 



changes of sign, and on the whole the potential 

 gradient is more often reversed than not. 



C. Snow. 



(8) Snow is sometimes positively and sometimes negatively 



charged. 



(9) In Simla positive electricity was in excess, while in 



Potsdam an excess of negative electricity was 

 observed. 



(10) A given weight of snow may be more highly charged 



than the same amount of rain, even in a thunder- 

 storm. 



(11) High values of the potential gradient, both positive 



and negative, occur during snowfall. 



In the above I have only stated results about which there 

 is no difference of opinion ; there are many others which are 

 almost as certainly true, but as there is still some doubt they 

 have been excluded. 



So far as I know there are at present only two theories 

 seriously put forward to account for the electricity of 

 precipitation. 



(A) The " influence " theory, first put forward by Elster 



and Geitel in 1885 *, and revised by them in 1913 f. 



(B) The " breaking drop " theory, put forward by the 



writer in 1909 { to explain the origin of electricity in 

 thunderstorms. 



I propose to consider each of these theories in the light of 

 the established facts detailed above. 



* Elster & Geitel, Wied. Ann. xxv. pp. 123-124 (1885). 

 f Elster & Geitel, Phys. Zeit. xiv. pp. 1287-1292 (1913). 

 % Simpson, Phil. Trans. Roy. Soc. A. ccix. pp. 379-413 (1909). 



