390 DE. G. C. SIMPSON ON THE ELECTEICITY OF 



in columns 4 and 6 the corresponding mean charges per cubic centimetre are tabulated. 

 Column 7 gives the ratio of the number of two minutes of positive rain to the number 

 of two minutes of negative rain. 



Two interesting results are shown in this table, the most important being that the 

 ratio of the number of positively charged falls to negatively charged falls increases as 

 the rainfall becomes more intense. In this respect it is important to notice that in 

 no case in which the rainfall exceeded 1 mm. in two minutes was a negative 

 charge associated with the rain. The second interesting result is that both with 

 positively and negatively charged rain the highest charges are carried down by light 

 rain, although with heavier rain than 0'028 cm. in two minutes the magnitude of the 

 charge does not appear to depend on the intensity of the rain. 



The general character of the rain in reference to its electrical state may now 

 be discussed, starting with the rain which accompanied thunderstorms. 



It has already been pointed out that a hard and fast line cannot be drawn between 

 rain which falls (luring thunderstorms and rain not connected with thunderstorms. Still 

 there can be no possible doubt that the rain connected with thunderstorms was more 

 highly charged than rain with which were associated few lightning discharges or none 

 at all. During some storms in July and August, which were not accompanied 

 by electrical effects, rainfall exceeding 0'070 cm. in two minutes occurred without giving 

 the slightest indication of any electrical charge, while on the other hand it sometimes 

 happened that during thunderstorms the rain was too light to be registered on 

 the rain-gauge, and yet charged up the receiver to a potential between 20 and 

 30 volts in two minutes. It is, however, important to point out that the most highly 

 charged rain did not always accompany the storms with the greatest amount of 

 thunder and lightning. Also, often during a thunderstorm the rain would continue 

 after the thunder and lightning had ceased, and yet be as highly, if not more highly 

 charged than during the violent electrical display. 



As a preliminary to discussing the general characteristics of negatively charged 

 rain, the remarkable storm of May 13, 1908, which has already been referred to, will 

 be considered. 



At about midday the weather appeared threatening and a violent thunderstorm 

 worked up soon after 13 hours. The thunder was very loud and near, and the 

 lightning vivid. At 13 hours 46 minutes the rain began to fall, but not very heavily. 

 The heaviest rain occurred at about 14 hours 36 minutes, the average rainfall being 

 then at the rate of 0'042 cm. in two minutes, and the greatest rate 0'070 cm. in the 

 same time. At 14 hours 48 minutes the rate of rainfall became less, and from then 

 until 17 hours 30 minutes steady rain continued at an almost constant rate of 

 0-014 cm. in two minutes. Unfortunately, during the storm it was impossible to 

 follow the changes very closely owing to the demands of other work, and hence it is 

 not known exactly at what time the thunder and lightning ceased ; there are reasons, 

 however, for believing that it did not continue long after 15 hours, and it is known 



