410 DR. G. C. .SIMPSON ON THE ELECTRICITY OF 



electricity. Now where the vertical current spreads out horizontally the stream 

 lines of the air have horizontal components. For the present purpose it does not 

 matter whether we consider that the vertical current spreads out uniformly on all 

 sides or is deflected in a certain direction. In either case the water accumulated at 

 the head of the current will be gradually moved horizontally until it reaches the 

 edge of the rapid vertical current, and there it will be able to escape by falling ; but 

 as it will necessarily take some time for any given mass of water to travel from the 

 centre to the outside of the ascending current, there will be time for considerable 

 breaking up before the drops actually fall. Thus the water carried up by the 

 ascending current will fall as positively charged heavy rain over one or other of the 

 edges of the ascending current. This is of course considering the case in its simplest 

 terms. As a matter of fact, the ascending current will vary in velocity, will have 

 its gusts and lulls just as a horizontal wind has. Such variations, however, will be 

 of great help in causing splashing, for LENARD* found that " the sudden contact of 

 already deformed drops with a quicker air stream is very favourable to the breaking 

 of the drops." Another consequence of the gusts and lulls in the ascending currents 

 will be the raising and lowering of the region in which the water is held in 

 suspension, and with this will follow rapid changes in the electrical field which may 

 possibly help to produce electrical discharges. 



The water which has become positively charged on the ascending current and then 

 fallen as rain will be the heavy rain which occurs in the centre of the thunderstorm : 

 the ' Platzregen " of the German authors. In view of this consideration, it will be 

 interesting to look at the results of actual measurements of rain electricity, to see if 

 the heavy rain which falls in the centre of a thunderstorm is positively charged. 

 There are four sets of measurements of rain electricity which can be used for the 

 purpose, viz., those of ELSTER and GEITKL'S first series of experiments, published in 

 the ' Wiener Berichte ' ; their second series, published in ' Terrestrial Magnetism and 

 Atmospheric Electricity ' ; those of WEISS, published in the ' Wiener Berichte ' ; and 

 finally those in this paper. 



These will be taken in the reverse order, and the Simla measurements considered 

 first. It has already been pointed out in the first part of this paper (p. 390), that in 

 every one of the 97 cases in which the rainfall equalled or exceeded 0112 cm. in 

 two minutes the rain was positively charged. 



WEISS f does not record any case of a thunderstorm, nor do rainfalls so large as 

 those just described occur in his observations ; but nevertheless he made observations 

 during two rain squalls (" Regenboe," Tables 12 and 14), and he states that in the 

 former " Platzregen " fell ; from his tables it will be seen that this excessive rain 

 was positively charged (Nos. 182-184 of Table 12). In the second "Regenboe" the 

 rain was positively charged throughout. 



* Loc. ci(., p. 256. 



t WEISS, ' Wien. Ber.,' vol. 115, p. 1285, 1906. 



