88 



Prnceedmrix of Ihr RniinI Iri-th Academi/. 



"Large" Rain. 



In the case of rain in which large drops were present we took some care 

 to note whether the large drops were accompanied by rain of the " fine " type. 

 That this distinction is necessary was clearly shown by what happened on 

 some occasions. We noticed occasions when much fine rain was clearly 

 accompanied by a relatively small number of large drops ; and the electro- 

 meter deflections changed qiiickly from positive to negative as the number of 

 large drops increased or diminished. We had little doubt from our observa- 

 tions of the rain and the electrometer-deflections that the " fine " rain was 

 as iisual negatively charged and the larger drops positively charged. Of 

 course it is quite impossible to decide on everj^ occasion whether the rain 

 should be classed as " large " rain or " mixed " rain ; but often it was clearly 

 of the " mixed " type, and such cases are summarized in a separate class. 



The charge on the rain consisting entirely of large drops was almost 

 always positive. Sometimes in the case of such rain a sudden change takes 

 place, and for a few minutes or less a negative charge is collected, followed by 

 a quick return to the positive again. 



The value of the positive charge on large rain is shown by the following 

 Table (I), which gives the percentage of the total discharges of the tipping- 

 buckets corresponding to charges between certain limits : — 





Table I 



Charge pei- c.c. in Electi-ostatii 



FTiitP. 



<-l 





•1 ~ -r. 





•r. - 1 . 





1 - 2 . 





2 - :! . 





3-4 . 





4 - .5 . 





Ti - 9 





Percentage. 



3 

 . 13 

 . 21 

 . 46 



6 



On almost half of the rain of this type examined the charge per c.c. was 

 between 1 and 2 electrostatic units. The highest charge obser\'ed was 

 9'2 units per c.c. 



As stated above, the charge per c.c. on the negatively electrified " fine " rain 

 was between the limits '06 and '24 units per c.c. 



On the occasions when a negative charge was observed on rain which could 

 not be classed as fine rain higher values were found, reaching 4-9 units per c.c. 

 on one occasion. 



