Thunderstorm Electricity. 639 



p. 121, 1885) that the intense electric fields o£ thunderstorms 

 might be due to electrostatic influence effects accompanying 

 the fall of larger drops through a cloud of smaller ones — a 

 weak initial electrical field being assumed. 



To what extent condensation upon the ions is likely to be 

 effective after the initial stages of the showers is a difficult 

 question into which I do not propose to enter. I have never 

 regarded it as at all likely that the supersaturation necessary 

 to cause condensation upon the positive ions could ever 

 occur in the atmosphere, as Elster and Geitel and others 

 have imagined. 



The question of the occurrence of supersaturation in the 

 atmosphere is of course fundamental in relation to the question 

 under discussion. In the article in 'Nature' quoted by 

 Mr. Simpson nothing was said about the condition of the 

 atmosphere immediately above the cloud. The argument 

 was intended to show how, even if the lower moisture-charged 

 layers were highly charged with dust-particles, a super- 

 saturated layer might still result, and the upper atmosphere 

 remain dust free. 



To make the matter clearer, let us assume that above a 

 lower dust- and moisture-charged layer we have a dry and 

 dust-free layer. (It may be remarked that a large mass of 

 air in the atmosphere can hardly have become dry otherwise 

 than by the condensation and subsequent precipitation of the 

 water from it — a process which is likely to remove from it 

 the dust-particles as well. It is of course possible that there 

 may be in the upper atmosphere a continual production of 

 dust-particles — under which title we may include all per- 

 sistent nuclei more efficient than the ordinary negative ion.) 

 If we imagine a local ascensional current to begin, the surface 

 of separation of the two layers will rise ; and when it has 

 reached the level corresponding to saturation for the lower 

 layer a cumulus cloud will begin to form. The upper surface 

 of this cumulus cloud marks appoximately the surface of sepa- 

 ration between the dry and moist layers. The transition 

 from the dry to the moist layer cannot be absolutely sudden 

 on account of diffusion ; in consequence condensation will 

 first begin somewhat below the upper limit of the dust- 

 charged air. As the air continues to rise the cloud will thus 

 at first grow T partly by additions to its upper edge, the newest 

 and smallest drops being at the top. If the transition layer 

 between the dry and moist air continues to ascend, a stage 

 will be reached when saturation extends to the highest level 

 reached by the dust-particles. Beyond this stage the cloud 

 will cease to grow by addition of new drops at its upper 



