84 Proceedings of the Roijul trish Academy. 



high, and 9i cms. in diameter. All this zinc, as well as the wooden box, is con- 

 nected to Earth. The receiver is thus protected from the Earth's electrostatic 

 field, from which it is furthermore shielded owing to the fact that the apparatus 

 is placed in a small quadrangle, where the potential gradient is small. The 

 receiver is connected by an insulated wire passing through an earthed metallic 

 tube to a Dolezalek electrometer in a building about 4 metres distant. The 

 receiver is supported on a tripod, from which it is insulated by paraftin wax T I. 

 It was found that this insulation kept very good, even when the humidity of the 

 air was high. Attached to the receiving vessel, and in electric connexion 

 with it, is a tipping-bucket arrangement K, of the ordinary kind, into which 

 the rain flows directly. This is adjusted to tip when 30 c.cs. of rain have 

 flowed into it. The water discharged at each tip of the bucket is caught in 

 a pan underneath, and flowing out through a pipe falls upon a little vessel V, 

 supported by a spring, and causes it to make contact with mercury, and thus 

 complete a circuit. The closing of the circuit causes a bell to ring close to 

 the observer, and thus the arrival of each 30 c.c. of rain iu the receiver is 

 signalled. The little vessel is perforated, so that after the rush of water 

 ceases, the strength of the spring restores it to its first position, and the 

 contact is broken. 



The charge brought down by the rain is measured by observing the 

 increase of potential of the receiving vessel by means of a Dolezalek 

 electrometer. In these experiments the capacity of the vessel, electrometer, 

 and connexions was 324 cms. The sensibility of the electrometer was 

 464 mm. scale-divisions per volt, and was very approximately constant 

 throughout the experiments. In general the charge on the rain was such 

 that it was necessary to increase largely the capacity of the electrometer 

 system by connecting a condenser. When a suitable capacity, depending 

 upon the degree of electrification of the rain, is connected to the electrometer, 

 the method of experiment resolves itself into noting the number of scale- 

 divisions through which the electrometer needle is deflected in the interval 

 between two discharges of the tipx^ing-buckets. The electrometer is connected 

 to Earth at each ring of the bell, and the deflection at the next ring noted. 

 Thus the charge per c.c. of rain can be directly determined. 



In the course of these experiments no attempt was made to use self- 

 recording apparatus. It was considered that more information could be 

 gained by careful observation of the type of rain on each occasion, and of 

 other attendant circmnstances. Further, by personal attention, we have been 

 able to observe the charge during exceedingly slight rainfalls as well as in the 

 case of heavier rain. While the method has the disadvantages of only dealing 

 with a portion of the total rainfall in any given period, and requiring much 



