Theory of Thunderstorm Electricity . 621 



cloud the innumerable water-drops will act as a kind o£ filter 

 and remove all the dust-particles from the air. The drops 

 at the top of the cloud will also lag behind the ascending air,, 

 and the air between them is likely to be dust-free. " Under 

 these conditions a dust-free layer will be formed above the 

 cloud, and will continually increase in vertical thickness.. 

 This layer will be saturated with moisture at its lower edge, 

 above this it will be supersaturated ; the amount of super- 

 saturation being greater near its upper limit, and depending 

 on the vertical distance through which the air has risen since 

 escaping from the cloud. Now to produce in air initially 

 saturated the supersaturation (approximately four-fold) 

 necessary to cause water to condense on negative ions, it is 

 sufficient to let the volume of the air increase adiabatically 

 to 1/25 times its initial value ; an expansion which will result 

 from an ascent of the air through a vertical distance of 

 2,500 metres, if we suppose the air on escaping from the cloud 

 to be at a temperature of 10° C. (at lower temperatures a 

 smaller elevation would suffice). Thus, when the air in the 

 uppermost layers of the supersaturated stratum has reached 

 a height of about 2,500 metres above the level at which it 

 escaped from the cloud, a sudden change will result ; con- 

 densation will there take place on the negative ions. The 

 thickness of the supersaturated stratum (*. e. the vertical 

 distance which the upper surface of this cloud has lagged 

 behind the air) when the condensation on the negative ions 

 begins, may vary greatly ; it may be very small if the drops 

 are small and the ascent of the air rapid ; it may amount to 

 nearly the whole 2,500 metres in the case where the drops 

 grow large enough to acquire a velocity relative to the air 

 as great as the upward velocity of the air, so that the upper 

 surface of the cloud has ceased to ascend.'' The drops which 

 condense on the negative ions at the top of the supersaturated 

 ^ayer will either fall at once as rain or remain in suspension 

 till thev have travelled into regions where the ascending 

 current is insufficient to support them. " In either case, it' 

 the drops fall through a supersaturated layer of some thick- 

 ness, they are likely to reach the ground as negatively charged 

 rain/' " The positive ions after being carried up out of reach 

 of the drops formed on the negative ions, will, under the 

 action of the electrical field produced by this separation, tend 

 to travel downward relatively to the air with a velocity of 

 the order of one centimetre per second for a field of 100 volts 

 per metre, as the measurements of Rutherford and others 

 have shown. After being- carried bevoud the region of 

 ascending air-currents, they will travel downwards towards 



