82 Proceedings of the Royal Irisli Academy. 



Simpson's observations are the most complete that have yet been made 

 upon this subject. His apparatus was self-recording, and measured the 

 charge of the rain per c.c. as well as per unit time. The sign of the potential 

 gradient was also automatically recorded. Simpson summarizes his results 

 as follows : — 



(1) During 71 per cent, of the time that charged rain fell the charge was 

 positive. 



(2) 75 per cent, of the electricity brought down by the rain was positive. 



(3) Light rain was more highly charged than heavy rain. 



(4) All rainfall that occurred at a greater rate than one mm. in two 

 minutes was positively charged. 



(5) The proportion of negative electricity brought down by the rain was 

 .slightly greater in the second than in the first half of the storms. 



(6) The potential gradient was more often negative than positive during 

 rain. 



(7) No relationship between the sign of the potential gradient and the 

 sign of the electricity of the rain could be detected. 



Simpson also investigated the electricity brought down by snow.' He 

 found that the proportion of positive to negative was about 3"6 to 1 ; but the 

 most striking result was that the average charge per gramme wa.s much 

 greater than in the ease of rain. 



Observations have been made in 1910 by Baldit,^ at Puy-en-Yelay. He 

 finds that positively charged rain falls more frequently than negatively 

 charged rain, and that an excess of positive electricity is brought down. 



It seemed to us important to have further observations of the charge on 

 rain, as the later and more complete work was not in agreement with earlier 

 work and ideas on the subject, but especially because the standard work by 

 Simpson was carried out during the thunderstorm season at Simla, so that his 

 results could not 'be assumed to hold under normal weather conditions. To 

 this end we ha^'e made observations of the electric charge on rain at University 

 College, Dublin. This paper deals chiefly with work started in March, 1911, 

 and carried on to the end of June, a period of exceptionally small rainfall. 

 The work was again taken up in October, and is being carried on during the 

 winter months. 



' Proc. Roy. Soc, Ixxxiii, 83, 1910, p. 394. 



- Ciiminunicatioii ii lu Societe meteoiologique, seaiice du Mars, 1911. Le Eadium, Aviil, 1911, 

 p. 153. 



