RAIN AND ITS OEIGIN IN THUNDERSTORMS. 



391 



for certain that it had ceased before 16 hours.* During the period at the commence- 

 ment of the storm, when the rain was moderately heavy and the electrical discharges 

 violent, the rain was positively charged, but from 15 hours 24 minutes to 17 hours 

 8 minutes, that is, during the steady rain without thunder and lightning, the rain 

 carried down a negative charge which at times exceeded 19 els. units per centimetre 

 of water, this being by far the greatest charge measured on any rain. This charge is 

 so great that the electrical force on the raindrops within a field of half the intensity 

 necessary to cause a lightning discharge would be equal to the force exerted by 

 gravity on the raindrops, so that it would be quite possible for fields to occur which 

 would actually cause such drops to rise against gravity. 



The noteworthy features of this highly negatively charged rain were that it occurred 

 after a violent thunderstorm, during which the rain had been positively charged, and 

 that the rate at which it fell was not great, but uniform and steady. These 

 characteristics of negatively charged rain which were so marked in this storm are 

 more or less traceable throughout all the storms investigated. 



It has already been pointed out that negatively charged rain does not occur when 

 the rate of the rainfall becomes large, and from Table V, p. 389, it will be seen that 

 the rainfall was less than 0'028 cm. in two minutes during 88 per cent, of the time 

 that negatively charged rain fell, while for positively charged rain this proportion was 

 only 64 per cent. Thus it would appear that negative electricity is, as a rule, brought 

 down by light rain. 



Negatively charged rain fell during all periods of the storms, and in some very rare 

 cases the whole rain was negatively charged ; but there appeared, from the data 

 collected, to be a tendency for negative 'electricity to be associated with the latter 

 half of a storm. In order to bring out the relationship, as many storms as possible 

 have been divided into four equal parts as regards time, and the occurrences of 

 positively and negatively charged rain in each quarter have been counted, with the 

 following result : 



TABLE VI. 



Quarter of the storm. 



1st 

 2nd 

 3rd 

 4th 



Percentage of time of rainfall during which 

 the charge measured was negative. 



From this it is seen that although more positive than negative rain fell in all periods 

 of the storms, the difference was least in the second half, or, in other words, the 



* This storm occurred before the coherer had been installed. 



